Disease and Defense2.2 Flashcards
1
Q
enterotoxin
A
a protein exotoxin released by a microorganism that targets the intestines.
2
Q
endotoxin
A
also known as LPS, are large molecules consisting of a lipid and a polysaccharide composed of O-antigen, outer core and inner core joined by a covalent bond; they are found in the outer membrane of Gram-negative bacteria, and elicit strong immune responses in animals.
3
Q
exotoxin
A
a toxin secreted by bacteria.
4
Q
para-aminobenzoic acid or PABA
A
PABA is an intermediate in the synthesis of folate by bacteria, plants, and fungi. Many bacteria, including those found in the human intestinal tract such as E. coli, generate PABA from chorismate by the combined action of the enzymes 4-amino-4-deoxychorismate synthase and 4-amino-4-deoxychorismate lyase. Plants produce PABA in their chloroplasts, and store it as a glucose ester (pABA-Glc) in their tissues. Humans lack the enzymes to convert PABA to folate, so require folate from dietary sources such as green leafy vegetables. Although some intestinal bacteria can synthesize folate from PABA and some E. coli can synthesize folate, this requires six enzymatic activities in folate synthesis which are not all done in the same bacteria. In humans, PABA is considered nonessential and, although it has been referred to historically as “vitamin Bx”, is no longer recognized as a vitamin.Sulfonamide drugs are structurally similar to PABA, and their antibacterial activity is due to their ability to interfere with the conversion of PABA to folate by the enzyme dihydropteroate synthetase. Thus, bacterial growth is limited through folate deficiency
5
Q
alanine racemase
A
this enzyme has one substrate, L-alanine, and one product, D-alanine. This enzyme participates in alanine and aspartate metabolism and D-alanine metabolism. It employs one cofactor, pyridoxal phosphate. At least two compounds, 3-Fluoro-D-alanine and D-Cycloserine are known to inhibit this enzyme. Bacteria can have one (alr gene) or two alanine racemase genes. Bacterial species with two genes for alanine racemase have one that is continually expressed and one that is inducible, which makes it difficult to target both genes for drug studies.
6
Q
D-ala-D-ala pentapeptide
A
the two substrates of this enzyme are UDP-Mur2Ac(oyl-L-Ala-gamma-D-Glu-L-Lys-D-Ala-D-Ala) and undecaprenyl phosphate, whereas its 3 products are UMP, Mur2Ac(oyl-L-Ala-gamma-D-Glu-L-Lys-D-Ala-D-Ala)-, and diphosphoundecaprenol. This enzyme belongs to the family of transferases, specifically those transferring non-standard substituted phosphate groups. his enzyme participates in peptidoglycan biosynthesis.
7
Q
transpeptidase
A
a bacterial enzyme that cross-links peptidoglycan chains to form rigid cell walls. In Gram-positive bacteria, the peptidoglycan molecules are cross-linked by a pentapeptide bridge, whereas, in Gram-negative bacteria, the peptidoglycan molecules are directly covalently bound to each other. The antibiotic penicillin irreversibly binds to and inhibits the activity of the transpeptidase enzyme by forming a highly stable penicilloyl-enzyme intermediate. Because of the interaction between penicillin and transpeptidase, this enzyme is also known as penicillin-binding protein (PBP).
8
Q
Methicillin-resistant Staphylococcus aureus (MRSA)
A
MRSA is any strain of Staphylococcus aureus that has developed, through the process of natural selection, resistance to beta-lactam antibiotics, which include the penicillins (methicillin, dicloxacillin, nafcillin, oxacillin, etc.) and the cephalosporins. Strains unable to resist these antibiotics are classified as methicillin-sensitive Staphylococcus aureus, or MSSA. The evolution of such resistance does not cause the organism to be more intrinsically virulent than strains of S. aureus that have no antibiotic resistance, but resistance does make MRSA infection more difficult to treat with standard types of antibiotics and thus more dangerous.
9
Q
vancomycin-resistant enterococci (VRE)
A
bacterial strains of the genus Enterococcus that are resistant to the antibiotic vancomycin. The mechanism of resistance to vancomycin found in enterococcus involves the alteration to the terminal amino acid residues of the NAM/NAG-peptide subunits, under normal conditions, D-alanyl-D-alanine, to which vancomycin binds. The D-alanyl-D-lactate variation results in the loss of one hydrogen-bonding interaction (four, as opposed to five for D-alanyl-D-alanine) being possible between vancomycin and the peptide. This loss of just one point of interaction results in a 1000-fold decrease in affinity. The D-alanyl-D-serine variation causes a six-fold loss of affinity between vancomycin and the peptide, likely due to steric hindrance.
10
Q
Production of penicillinase enzyme via a plasmid
A
Induced in the presence of penicillin. May be transmitted by bacteriophages (transduction. Major problem with staphylococcus (MSSA). NOTE: β lactamase is generic term for enzymes that hydrolyze β-lactams for Penicillinases and Cephalosporinases
11
Q
Alterations in penicillin-binding proteins
A
Responsible for methicillin resistance in staphylococci (MRSA)
12
Q
Inability to penetrate into the bacterial cell in penicillins
A
Penicillin G can’t enter many gram neg bacteria (e.g., Pseudomonas) due to presence of outer membrane
13
Q
“Escape” or Persisters Resistance to Penicillins
A
Metabolically inactive organisms or “L” forms can survive in a hypertonic environment like the kidney
14
Q
penicillin absorption
A
Oral absorption varies depending on acid stability. Penicillin G poor and unreliable. Penicillin V and Amoxicillin excellent. Piperacillin and Ticarcillin and IV only. IM absorption dependent on salt form. Rapid from aqueous solutions. Delayed from suspensions (procaine – benzathine). Use against organisms susceptible to low but sustained levels of Pen G (syphilis- endocarditis)
15
Q
Microbial toxins
A
are macromolecular products of microbes
that cause harm to susceptible animals by altering cellular structure or function. They are very potent, and the clostridial neurotoxins (botulinum and tetanus toxins) are the most toxic biological substances known. Some toxins cause the major manifestations of specific
diseases (for example: in botulism, cholera, diphtheria, whooping cough, scalded skin syndrome, scarlet fever, tetanus, or toxic shock syndrome). Other toxins contribute to pathogenesis without causing unique signs or symptoms (for example, pneumolysin). Toxin-mediated diseases cause significant morbidity and mortality, particularly in developing countries.
16
Q
Traditional Methods to Show that a Specific Toxin Has a Role in Pathogenesis
A
Show that purified toxin causes the same symptoms or signs as infection by the toxin-producing microbe. Show that antitoxin prevents disease caused by the toxin-producing microbe. Show that virulence of individual bacterial strains correlates with the amount of toxin that they produce. Show that nontoxinogenic mutants are avirulent and that virulence is restored if the microbe regains the ability to produce toxin.
17
Q
Molecular Version of Koch’s Postulates
A
Show that the phenotype or property to be investigated (e.g., toxin production) is associated with a pathogenic species or with pathogenic strains of a microbe. Show that inactivation of a specific gene(s) that encodes the putative virulence factor causes a measurable decrease in virulence of the microbe. Show that replacement of the mutated gene by the wild type allele restores virulence of the microbe to the original, wild type level.
18
Q
Bacterial protein toxins
A
are usually heat-labile, immunogenic, and neutralized by specific antibodies. They were originally called “exotoxins” to indicate that they were found outside the bacterial cells. Some (like diphtheria toxin) are actually secreted into the culture medium. Others (like botulinum toxin) are released by lysis of the bacteria.
19
Q
Lipopolysaccharides (LPS)
A
gram negative bacteria were first called “endotoxin” to indicate their association with bacterial cells. LPS is an example of a pathogen-associated molecular pattern (PAMP) that is recognized by the innate immune system and elicits host responses by a pathway that involves LPS binding protein, CD14, TLR4, and other signal transduction molecules. Low LPS doses activate macrophages, B-cells and the alternative complement pathway to cause fever, production of acute phase reactants, polyclonal antibody synthesis, and inflammation. High doses of LPS cause shock and disseminated intravascular coagulation. Many of the biologic effects of LPS are mediated by cytokines. LPS will not be discussed further in this lecture.
20
Q
Toxins that facilitate spread of microbes through tissues.
A
Some toxic enzymes break down extracellular matrix or degrade debris in necrotic tissue (e.g., hyaluronidase, collagenase, elastase, deoxyribonuclease, and streptokinase), thereby enhancing spread of microbes.
21
Q
Toxins that damage cellular membranes
A
Most membrane-damaging toxins kill target cells. Many are called hemolysins, because it is easy to detect their action on erythrocytes. Usually these toxins also damage other cells and are more accurately called cytolysins. Many membrane-damaging toxins insert into membranes and assemble into multimeric complexes that form pores, thereby causing lysis of target cells. Others, such as lecithinases, degrade specific cell membrane components and disrupt the integrity of the membranes.
22
Q
Toxins that stimulate cytokine production
A
The pyrogenic exotoxins include erythrogenic (scarlatinal) toxins of Streptococcus pyogenes and the enterotoxins and toxic shock syndrome toxin (TSST-1) of Staphylococcus aureus. They are involved in scarlet fever, food poisoning, and toxic shock syndrome. The pyrogenic exotoxins belong to a larger class of molecules known as superantigens.
23
Q
superantigens
A
are the most potent known T cell activators. They act by binding both to major histocompatibility (MHC) class II molecules on antigen-presenting cells and to specific Vβ chains on T cells at a site that is different from the antigen-binding site, and they activate much larger numbers of T cells than any specific antigen does. Superantigens stimulate excessive production of cytokines (including interleukin-2, interferon gamma, and others), thereby causing pathologic effects.
24
Q
Toxins that inhibit protein synthesis
A
These toxins inhibit protein synthesis irreversibly and cause death of intoxicated host cells. Examples include diphtheria toxin, Pseudomonas aeruginosa exotoxin A, shiga toxins of shigella dysenteriase, and E coli.
25
Diphtheria toxin and Pseudomonas aeruginosa exotoxin A
inactivate elongation factor 2 (EF-2), which is required for peptide chain elongation. It is a toxin that inhibits protein synthesis. They are both ADP ribosyltransferases that transfer adenosine diphosphate ribose (ADP-ribose) from nicotinamide adenine dinucleotide (NAD) to a modified histidine residue called diphthamide on EF-2, thereby inactivating EF-2 in the cytoplasm.
26
Shiga toxins of Shigella dysenteriae and E. coli
and the plant
toxin ricin, are highly specific RNA N-glycosidases that remove one particular adenine residue from the 28S RNA of the 60S ribosomal subunit, thereby inactivating the ribosomes. It is a toxin that inhibits protein synthesis.
27
Toxins that modify intracellular signaling pathways
These toxins alter specific cellular functions and may or may not cause cell death. Examples include heat-labile enterotoxins of Vibrio cholerae and Escherichia
coli, Pertussis toxin, Heat-stable enterotoxin I (ST-I) of E. coli, Anthrax edema factor (EF), Anthrax lethal factor (LF), and Clostridium difficile toxins A and B.
28
Heat-labile enterotoxins of Vibrio cholerae and Escherichia
| coli
are ADP ribosyltransferases that increase cell membrane-associated adenylate cyclase activity by ADP-ribosylating and activating the α subunit of the stimulatory Gs regulatory protein of the cyclase complex. Increased intracellular cAMP in small intestinal enterocytes causes active chloride secretion and results in secretory diarrhea. It is a type of toxin that modifies intracellular signaling pathways.
29
Pertussis toxin
an ADP ribosyltransferase that increases cell membrane-associated adenylate cyclase activity by ADP-ribosylating and inactivating the α subunit of the inhibitory Gi regulatory protein of the cyclase complex. Increased intracellular cAMP causes tissue- specific effects. It is a type of toxin that modifies intracellular signaling pathways.
30
Heat-stable enterotoxin I (ST-I) of E. coli
activates cell membrane-associated guanylate cyclase. Increased intracellular cGMP in enterocytes also causes secretory diarrhea. It is a type of toxin that modifies intracellular signaling pathways.
31
Anthrax edema factor (EF) from Bacillus anthracis and adenylate cyclase toxin from Bordetella pertussis
pertussis are adenylate cyclases that enter target cells, cause intracellular cAMP to increase, and produce cAMP-dependent effects. Their enzymatic activity requires activation by calmodulin and calcium, which are provided by the target cells. It is a type of toxin that modifies intracellular signaling pathways.
32
Anthrax lethal factor (LF)
an endopeptidase that cleaves several MAP kinase kinase proteins and inactivates their function in signal transduction. It is not yet know precisely how inactivation of this signal transduction pathway contributes to the lethal effects of LF. It is a type of toxin that modifies intracellular signaling pathways.
33
Clostridium difficile toxins A and B
are glucosyl transferases that alter the actin cytoskeleton of target cells by transferring glucose from UDP-glucose to several Rho family GTPases (including Rho, Rac and Cdc42), thereby inactivating them. It is a type of toxin that modifies intracellular signaling pathways.
34
Toxins that inhibit release of neurotransmitters
Includes botulinum toxin tetanus toxin.
35
Botulinum toxin
(7 antigenic types, A-G) causes flaccid paralysis of skeletal muscles by inhibiting release of acetylcholine at myoneural junctions. Types A, B and E most often cause disease in humans.
36
Tetanus toxin
(1 antigenic type) causes sustained muscular contraction (spastic paralysis/tetany) of skeletal muscles by inhibiting the release of neurotransmitter from inhibitory interneurons in the spinal cord.
37
Zinc dependent endopeptidases
Tetanus toxin and the botulinum toxins are zinc-dependent endopeptidases that inactivate specific SNARE proteins required for neuroexocytosis [VAMP (also called synaptobrevin), the 25 kDa synaptosome-associated protein (SNAP-25), and syntaxin]. Each toxin cleaves one specific protein at one specific site, and the individual serotypes of botulinum toxin differ in specificity from one another.
38
Therapeutic use for botulinum toxin
Botulinum toxin is used therapeutically to treat several focal dystonias and involuntary movement disorders, including strabismus and blepharospasm. It is also widely used for cosmetic effects.
39
Mechanisms and Pathways for Entry of Toxins into Cells
Toxins that act extracellularly or on plasma membranes are diverse
in structure and function. Their specificity is usually determined directly by the target of their action. Toxins with intracellular targets must cross the plasma membrane before they can exert their toxic effects.
40
Toxins with intracellular targets
They usually are bi-functional proteins with separate domains or subunits designated A (for active) and B (for binding). Susceptibility or resistance to toxins is often determined by presence or absence of receptors on the target cells. They typically use normal membrane constituents as receptors. Some toxin receptors are proteins: e.g., heparin-binding EGF-like growth factor precursor is the receptor for diphtheria toxin, and α2- macroglobulin receptor/low-density lipoprotein receptor-related protein [LRP] is the receptor for P. aeruginosa exotoxin A. Although diphtheria toxin and P. aeruginosa exotoxin A have identical intracellular actions, they act on different cell types because they utilize different cellular receptors. Some toxin receptors are glycolipids: e.g., ganglioside GM1 is the receptor for cholera toxin and E. coli heat-labile enterotoxins, and glycolipid Gb3 is the receptor for Shiga toxin. They usually enter target cells by endocytosis. Toxin receptor complexes enter endocytic vesicles and traffic to appropriate intracellular destinations. The active portion of the toxin is translocated to the cytosol to interact with its target. Diphtheria toxin, anthrax toxin protective antigen, botulinum toxin and tetanus toxin each have a translocation domain that changes its conformation in response to acidification of endosomes, inserts into the endosomal membrane to form a pore, and promotes translocation of the active component of the toxin to the cytosol. In contrast, Shiga toxin, cholera toxin, E. coli heat-labile enterotoxin, and pertussis toxin do not have a translocation domain. Instead, each of them trafficks via the retrograde pathway from endosomes, through the Golgi network, to the endoplasmic reticulum (ER). In the ER, the active component of each of these toxins is released from the holotoxin and is retro-translocated to the cytosol by a translocon of the ER-associated degradation (ERAD) pathway of the host cell. Subsequent intoxication of the cell results from the action of the active component of the toxin on its intracellular target.
41
Antitoxic antibodies (antitoxins)
bind to toxins and prevent their toxicity (neutralization). Antitoxins usually do not prevent infection by the toxin-producing bacteria or reverse toxic effects after the toxin has entered host cells.
42
Toxoids
are derivatives of toxins that retain immunogenicity but lack toxicity. They are used as vaccines for long term protection against toxin-mediated diseases.
43
Passive immunization
the administration of antibodies to a patient to provide immediate but temporary protection against a toxin or infectious agent. The duration of immunity is limited by degradation of the antibodies in the patient.
44
Active immunization
involves administration of toxoid to a patient in order to elicit production of specific anti-toxic antibodies. A primary series of immunizations and periodic booster doses are required to achieve and maintain protective levels of antitoxin. Active immunity can persist for many years because of immunologic memory.
45
Immunotoxins
"Immunotoxins" (and "hormonotoxins") are hybrid molecules consisting of a toxin fragment that lacks the receptor-binding domain of the native toxin (derived, for example, from diphtheria toxin, exotoxin A, or ricin) and that is linked (by chemical conjugation or as recombinant fusion protein) to a ligand (such as a monoclonal antibody or a "single-chain" antibody [in the case of an immunotoxin] or a hormone or its receptor-binding domain [in the case of an hormonotoxin]) that exhibits binding specificity for a specific receptor than is different from the receptor for the native toxin. The rationale is enable the immunotoxin or hormonotoxin to bind to cells that express that alternative receptor and intoxicate them by delivering the toxic fragment of the native toxin. Many immunotoxins are designed to kill tumor cells that display a tumor-specific receptor but not to kill normal cells that lack that receptor. Immunotoxins are being tested as potentially valuable therapeutic agents for treatment of specific cancers, autoimmune diseases, and other disorders.
46
Mechanisms of Action in Antimicrobial Pharmacotherapy
The mechanism of antimicrobial drug action identifies the DRUG TARGET. Knowledge of the drug target provides insight into: The relative degree of Selective Toxicity, potential mechanisms for antimicrobial drug Resistance, and whether the mechanism is Bactericidal or Bacteriostatic
47
Pharmacokinetics of antimicrobial pharmacotherapy
Knowledge of absorption, distribution, and elimination (metabolism and excretion) aids in selection of the proper route and timing of administration of antimicrobial agent. Also allows for consideration of dosage adjustments when gastrointestinal / kidney / liver problems are present.
48
Cocci gram positive bacteria
Streptococci (pneumoniae, pyogenes, viridans), Staphylococci (aureus: MSSA vs MRSA), Enterococci (faecium, faecalis)
49
Cocci gram negative
Neisseria (gonorrheae)
50
Rods gram negative
H. influenzae, E. coli, Klebsiella, Pseudomonas aeruginosa
51
Anaerobes gram positive rod
Bacteroides fragilis
52
Mechanisms of action for antimicrobial chemotherapy
disrupting cell wall synthesis (vancomycin, bacitracin, penicillins, cephalosporins, monobactams, cerbapenems), folic acid metabolis (trimethoprim, sulfonamides), cell membrane (polymyxins and daptomycin), protein synthesis (tRNA) (mupirocin and linezolid), protein synthesis (30s inhibitors) (tetracycline, streptomycin, gentamicin, tobramycin (aminoglycoside), and amikacin), protein synthesis (50s inhibitors) (erythromycin (macrolides), chloramphenicol, clindamycin, and streptogrmins), DNA directed RNA polymerase (rifampin) an DNA gyrase (quinolones).
53
Inhibition of synthesis or damage to cell wall
: targets include stage 1 alanine racemase, stage 2 D-ala-D-ala pentapeptide, stage 3 transpeptidase.
54
Stage 1 alanine racemase
cycloserine; enolpyruvate transferase: fosfomycin
55
Stage 2 D-ala-D-ala pentapeptide
vancomycin; bactoprenol lipid carrier: bacitracin
56
Stage 3 transpeptidase
penicillins, cephalosporins, monobactams, carbapenems
57
Inhibition of synthesis or damage to cell membrane
targets are membrane components (daptomycin and polymixin B).
58
Modification of synthesis or metabolism of nucleic acids
targets include DNA gyrase, RNA polymerase, and DNA
59
Antibiotics targeting DNA gyrase
fluroquinolones
60
Antibiotics targeting RNA polymerase
rifampin
61
Antibiotics targeting DNA
metronidacole and nitrofurantoin
62
Inhibition or modification of protein synthesis
targets include 30s and 50s ribosome and Isoleucyl-tRNA synthetase
63
Antibiotics targeting 30S ribosome
aminoglycosides, tetracyclines
64
Antibiotics targeting 50S ribosome
clindamycin, macrolides, chloramphenicol, streptogramins
65
Antibiotics targeting Isoleucyl-tRNA-synthetase
mupirocin
66
Modification of intermediary metabolism
interrupts metabolism of folate, targets include dihydropteroate synthase and dihydrofolate reductase.
67
Antibiotics targeting Dihydropteroate synthase
sulfonamides
68
Antibiotics targeting Dihydrofolate reductase
trimethoprim
69
Selective toxicity of antibiotics
Fundamental feature of antibiotic therapy as the effects of antimicrobial agents should be exerted selectively on microbe and not the host. No “perfect” antibiotics exist. Biochemical differences between the pathogen target and the host must be discovered and appropriately exploited.
70
Inhibition of a metabolic pathway found in bacteria but not in humans
Folate metabolism: Bacteria must synthesize folate intracellularly, while mammalian cells
can take up folate from the environment
71
Pathway exists in both bacteria and humans, but differences in enzyme structure
Protein synthesis: Bacterial ribosome consists of 30S and 50S ribosome, while
mammalian ribosome subunits are 40S and 60S. Nucleic acid synthesis: DNA gyrase (bacteria) vs topoisomerase (humans); RNA
polymerase is structurally distinct in bacteria
72
Macromolecular structure does not exist in humans
Cell wall synthesis: Peptidoglycan component does not occur in eukaryotes
73
Macromolecular structure differs between microbes and humans
Fungal cell membrane: Ergosterol is the major constituent of fungal membranes vs cholesterol in mammalian membranes
74
Natural (intrinsic) Resistance
Microbes lack a susceptible target for drug action. E.g., fungal cell walls do not contain peptidoglycans and mycoplasma do not have cell walls at all, thus they are naturally resistant to penicillins. Pseudomonas aeruginosa is intrinsically resistant to many antibiotics because they cannot cross its outer membrane.
75
Escape resistance
Microbes are sensitive and antibiotic reaches target BUT organism “escapes” the consequences due to availability of purines, thymidine, serine, methionine released from purulent infections (sulfonamide resistance) or failure to “lyse” due to lack of osmotic pressure difference (penicillin resistance). Emphasizes important role for surgical drainage procedures if practical.
76
Acquired Resistance
Selective pressure (i.e., antibiotic administration) produces successive generations of organisms with biochemical traits that minimize drug action. Two modes: Mutational (chromosomal) resistance and Plasmid mediated resistance
77
Mutational (chromosomal) resistance
[rate: 1 in 107-1012]: Chromosomal effect varies in degree with respect to location, type, and biological consequence of mutation. Multiple steps (generations) must occur for appreciable resistance as each succeeding generation becomes slightly more resistant if allowed to survive, thus proper dosing and duration of antibiotic therapy prevents survival of slightly resistant strains.
78
Plasmid mediated resistance
Plasmids are extrachromosomal pieces of circular DNA carrying genetic information
that can confer antibiotic resistance to the organism. This is a clinically important source of multiple drug resistance that can emerge
during a single course of treatment. Nonpathogenic coliform bacteria (gram negative) are a large reservoir for plasmid- mediated transfer of antibiotic resistance to pathogenic organisms; can code for resistance to multiple drugs (MDR gene) via protein that transports antibiotic out of cell. Exchange of genetic information among bacteria occurs by several mechanisms: Conjugation between two physically attached bacteria with exchange of plasmid DNA containing resistant determinant.
Transduction with virus (bacteriophage) carrying resistance determinant R to bacteria. Transformation - ability of certain bacteria to pick up free DNA from the environment.
79
Major mechanisms of bacterial resistance to antibiotics
includes altered targets, enzymatic destruction, alternative resistant metabolic pathway, decreased entry, and increased efflux.
80
Penicillin-binding proteins
[MRSA, S. pneumoniae, Enterococci) creates resistance in β-lactam antibiotics. Altered targets resistance or receptors to which the antibiotic cannot bind.
81
Altered DNA gyrase
[S. aureus, Pseudomonas species] can become resistance to Fluoroquinolones.
82
Peptidoglycan side chain alterations
[Enterococci (VRE), Staphylococci (VRSA)] leading to resistance against Vancomycin
83
50S ribosome methylation
[Strep-, Staph-, Enterococci] leading to resistance against Macrolides, Clindamycin
84
β-lactamase
produced by some bacteria that provide resistance to β-Lactam antibiotics like penicillins, cephamycins, and carbapenems (ertapenem). [S. aureus, P. aeruginosa, Bacteroides, Enterococci] leads to resistance against β-lactam antibiotics
85
Acetyl-/phospho-/adenylyltransferases
Enterococci] can become resistant to Aminoglycosides
86
Acetyltransferase
Staphylococci, Streptococci, Neisseria] can become resistant to Chloramphenicol
87
Overproduction of PABA or thymidine nucleotides
allows Streptococci to become resistant to Sulfonamides
88
Decreased entry (natural resistance)
β-lactam antibiotics [Pseudomonas aeruginosa] Fluoroquinolones [Pseudomonas species] Aminoglycosides [E. coli, Pseudomonas]
89
Increased efflux (multi-drug resistance may be encoded by single gene)
Tetracyclines [Streptococci, Staphylococci, Enterococci] Fluoroquinolones [Pseudomonas species]
Macrolides
90
Resistance can be minimized by
Only using antibiotic when need is established. Selecting antibiotic on basis of susceptibility tests. Using adequate concentration and duration to prevent emergence of first and second step mutants
91
Bactericidal vs. Bacteriostatic agents
Bactericidal (organisms are killed) vs bacteriostatic agents (organisms are prevented from growing). For a given antibiotic, whether or not it has a -cidal or a -static action is determined by its mechanism of action (target), concentration achieved in vivo, and the specific microorganism.
92
Bactericidal mechanisms
inhibition of cell wall synthesis, disruption of cell, disruption of cell membrane function, interference with DNA function synthesis. Bactericidal agent preferred in severe infections (assuming sensitive organism, drug distribution, and drug safety are compatible). Bactericidal agents act more quickly and their action is often irreversible with a sustained effect after the drug is eliminated from the blood. Bactericidal agents can compensate for patients with an impaired host defense (diabetes, agammaglobulinemia, immunosuppressive drugs, AIDS, asplenia). Bactericidal agents are required for treatment of infections in locations that are not accessible to host immune system responses (e.g., endocarditic vegetations and cerebrospinal fluid).
93
Bacteriostatic mechanisms
inhibition of protein synthesis (exception is aminoglycosides that end in –cidal) and inhibition of intermediary metabolic pathways.
94
Antimicrobial pharmacokinetics
Antimicrobial pharmacotherapy requires that the antibiotic reach the site of infection in sufficient concentrations (MIC) and once there remain active for a sufficient duration to be effective. Thus, in addition to a consideration of the pharmacodynamics of the antibiotic (antimicrobial activity against the specific infectious microorganism), it is also necessary to understand the pharmacokinetic properties: absorption from route (site) of administration, distribution to site of infection, elimination from body (hepatic or renal) as related to duration of antimicrobial activity.
95
Antimicrobial Absorption.
Provides information on the route of administration necessary for anti-infective effectiveness of the antibiotic. For most infections it is necessary to achieve adequate concentrations in the systemic circulation and most commonly this is accomplished via the oral or intravenous route. Certain infections can be managed with local application of the antibiotic, be it topically to the skin or mucous membranes for dermatological infections or oral administration of non-absorbable drugs for treatment of GI tract infections. The oral route has the advantage of ease of administration, patient acceptance, and lower cost while disadvantages can include GI upset or diarrhea due to alteration of intestinal flora, incomplete or lack of absorption for some drugs, and unsuitability in patients who are npo (nothing by mouth). “Take on empty stomach”: recommended when the antibiotic is unstable to the increased gastric acidity that occurs when food is in the stomach. “Take with food or meal”: recommended when the drug is acid stable but may be irritating to the stomach. The intravenous route is necessary for some drugs or some patients and has the advantage of providing the most rapid and predictable plasma levels when treating patients with life-threatening infections. Disadvantages associated with the IV route are the greater training needed, the greater expense, and the requirement for strict aseptic conditions. To reduce expense and complications seen with IV administration of antibiotics in hospitalized patients, efforts are being made to switch the patients to oral antibiotics whenever and as soon as possible (for certain antibiotics with excellent oral bioavailability [> 90%]).
96
Antimicrobial distribution of CNS
Most antibiotics distribute well to tissues outside the central nervous system, but vary substantially in their ability to cross blood brain barrier. Thus, penetration into the CNS is a necessary property of drugs that will be effective in treating CNS infections such as meningitis.
97
Antibiotics that readily enter the CSF
chloramphenicol, sulfonamides (trimethoprim), cephalosporins (3rd/4th), and rifampin (metronidazole).
98
Antibiotics that enter CNS with inflammation
penicillins, vancomycin, ciprofloxacin, and tetracycline.
99
Antibiotics that enter the CSF poorly
aminoglycosides, cephalosporins (1st/2nd), erythromycin, and clindamycin.
100
Antibiotics distribution to fetus
Adverse effects may occur in the fetus due to antibiotics that can cross the placental barrier. A general rule of thumb is that antibiotics that can be given orally, i.e., have the ability to cross the gastric mucosal barrier, can also cross the placenta and have the potential to harm the fetus. Drugs that should be used with caution or avoided during pregnancy include: aminoglycosides (D), chloramphenicol (C), fluroquinolones (C), metronidazole (1st trim), tetracyclines (D), and voriconazole (D).
101
Beneficial accumulations selective distributions/ accumulation
include clindamycin into bone (advantageous for treatment of osteomyelitis), concentration of macrolides into pulmonary cells (advantageous in upper respiratory infections), accumulation of tetracyclines into gingival crevicular fluid and sebum (advantageous in periodontitis and acne, respectively), rapid excretion of nitrofurantoin into urine (beneficial in urinary tract infections).
102
Selective accumulations that can increase the potential for toxicity
include aminoglycoside binding to cells of the inner ear and renal brush border resulting in an increased tendency for ototoxicity and nephrotoxicity or tetracyclines binding to Ca++ in developing bone and teeth resulting abnormal bone growth and brownish tooth discoloration in the fetus or young children.
103
Antimicrobial elimination
Knowledge of the route of antibiotic elimination is critical to the safe and effective use of antibiotics and at a minimum, one should know the major organ of elimination for antibiotics, either the kidneys or the liver.
104
Renal Excretion of Antimicrobial
Knowledge of those antibiotics drugs that are eliminated by renal excretion alerts
one to the possibility that renal dosing may be necessary in patients with kidney dysfunction. Renal dosing is the process whereby the dose and/or frequency of administration of the antibiotic are adjusted based on the patient’s renal function. Renal status is routinely monitored by measurement of serum creatinine (SCr) and estimation of creatinine clearance (CrCl).
105
Hepatic Metabolism of Antimicrobial
Knowledge of those antibiotics eliminated by drug metabolism alerts one to the possibility of metabolic drug-drug interactions, interpatient differences in metabolic rates (genetic polymorphisms), or hepatotoxic antibiotic actions. Unlike renally eliminated antibiotics, there is no lab value that gives a good estimate of the liver’s ability to metabolize antibiotics. Generally, antibiotics that require hepatic metabolism are simply avoided in patients with liver dysfunction if at all possible.
106
Duration of Antimicrobial Activity
Knowledge of the half-life of an antibiotic provides information regarding how often the drug will have to be administered to maintain the antimicrobial effect. Renal excretion and hepatic metabolism are the primary biologic processes that determine the plasma half-life of an antibiotic and the time that plasma levels remain above the MIC for a particular organism (its duration of action). Duration of therapy, along with the dose and dosage interval, can affect efficacy of therapy. If duration too short OR dose too low it can lead to resistance can develop as well as,
recurrence of infection. If duration too long, than superinfection more likely. If dose too high, than dose-related toxicities more likely to occur. For some antibiotics (aminoglycosides and fluoroquinolones), they continue to kill or inhibit growth of bacteria for several hours after the concentration of the drug falls below the MIC. This is known as the post-antibiotic effect and enables certain antibiotics to be given less frequently than would be predicted by their half-lives. Some antibiotics kill bacteria faster when given in doses that result in higher plasma concentrations, a property called concentration-dependent killing.
107
Clinical uses of antimicrobial
In hospitalized patients, culture and sensitivity results are often available within 24 hours, making the
selection of an antibiotic that will eradicate the microorganism causing disease less difficult. In the treatment of infections in the outpatient setting, culture and sensitivity data is infrequently available and antibiotics are commonly prescribed empirically based on symptoms, anatomic site, local
patterns of infections, and patient demographics.
In both settings, knowledge of antibiotic pharmacokinetics and toxicities is essential for selecting appropriate antibiotic for infection in a specific anatomic location in an individual patient.
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Spectrum of Antibacterial Activity
The antibacterial spectrum of antibiotics can be imprecisely categorized into narrow spectrum (effective against either gram positive or gram negative), extended spectrum (effective against gram positive and gram negative), and broad spectrum (effective against gram positive, gram negative, and atypical organisms). Narrow spectrum antibiotics are often most effective on susceptible organism; less disturbance of host flora. Broad spectrum antibiotics can sacrifice efficacy for greater scope of activity for initial empiric coverage; more likely to cause superinfections. Acute severe infections should be treated with broad spectrum aggressive antibiotic therapy (target empiric therapy to likely pathogens and local antibiogram) with a switch to narrow spectrum coverage as soon as possible (target definitive therapy to known pathogens and antimicrobial susceptibility test results).
109
Antibacterial with narrow spectrums
aminoglycosides, penicillinase- resistant penicillins, clindamycin, vancomycin, metronidazole, penicillin G,V.
110
Antibacterial with extended spectrums
aminopenicillins (Amox-amp), cephalosporins, fluoroquinolones (cip, levo), carbapenemas.
111
Antibacterial with broad spectrum
macrolides, chloramphenicol, luroquinolones (moxi, gemi), sulfonamides, tetracyclines, trimethoprim.
112
Antimicrobial Drug Combinations
Indications for use include: Empiric treatment of severe infections of unknown etiology (until organism identified). Mixed infections [e.g., oral or intraabdominal infections]. Delay or prevent the emergence of resistant strains [e.g., tuberculosis and AIDS]. Obtain synergistic effect [e.g., penicillin plus an aminoglycoside against enterococci]
113
Direct Toxicity
Antibiotic effect on microbes affects host cellular processes (lack of selective toxicity). Varies with specific drugs and concentrations (at target and non-target sites). Can range from mild to
life-threatening. Usually involves GI tract (nausea and vomiting, diarrhea, pseudomembranous colitis), liver
(hepatotoxic metabolites, which can lead to liver failure), kidney (direct toxic effects on renal cells from drugs excreted by kidneys), nervous system (irritant properties and inherent neurotoxicity), blood and blood forming system (bone marrow depression).
114
Indirect Toxicity
Allergic reactions, hypersensitivity. “Salt” effects. Due to salt administered with the antibiotic, not the antibiotic itself (Na+, K+ salts of
penicillin, estolate salt of erythromycin). Drug-drug interactions. Antibiotics may alter activity of CYP450 drug metabolizing enzymes
115
Disturbances of Host Microflora
(Superinfection).Disturbances of ecological balance of microbial community. Allows overgrowth of normally
suppressed pathogenic organism. Pseudomembranous colitis due to Clostridium difficile overgrowth can be life-threatening. More commonly associated with broad spectrum antibiotics. Increased incidence noted if: age < 3 y/o
or > 50 y/o, pulmonary disease (non-tuberculosis), treatment of prolonged duration.
116
Host Factors
Age. Very old and very young have more susceptibility to toxicities. Pregnancy / nursing mothers. Need to also consider harmful effects on fetus or infant. Drug hypersensitivity. Patient allergies may preclude use of otherwise effective agent
117
Structure of penicillins
cysteine and valine condensation product. The is a thiazolidine ring that is fused to beta-lactam ring and side chain. Also has allergenic sulfur atom.
118
Production of penicillin
via fermentation. Semi-synthetic penicillins: Hydrolysis of side-chain with bacterial amidases to produce 6-amino-
penicillanic acid followed by chemical addition of side chains, using acyl chlorides. Compounds with different “R” groups have different properties including those that can:
Increase acid stability (in GI tract),
Decrease renal excretion, increase metabolic stability, Minimize bacterial resistance (β-lactamase or amidase), Increase antibacterial spectrum by increasing bacterial penetration. Early dosage of penicillin expressed in units. With advent of pure penicillins, this measure of penicillin quantity is outmoded. Used only for penicillin G. Amount of all other penicillins stated in mg. [1 mg Penicillin G = 1667 units OR 250 mg = 400,000 units]
119
Bacterial Cell Wall Synthesis
Stage 1: Synthesis and assembly of cell wall subunits occurring in the cytosol (inhibited by fosfomycin [1] and cycloserine [2]). Stage 2: Linear polymerization of subunits occurring at cell membrane (inhibited by bacitracin [3] and vancomycin [4]). Stage 3: Cross-linking of peptidoglycan polymers occurring at the cell wall (inhibited by penicillins, cephalosporins [5]). Conformation of penicillin is similar to that of D-ala-D-ala, the terminal portion of the pentapeptide of N-acetylmuramic acid peptide that cross-links with the pentaglycine portion of an adjacent N-acetylmuramic acid peptide.
120
Mechanism of action of penicillins
Penicillins are bactericidal to growing organisms. Lysis of spheroplasts depends on osmotic pressure difference between inner/outer environments. Satisfactory explanation for lysis is lacking. Action of Penicillin G mainly confined to gram-positive organisms and gram-negative cocci, some spirochetes. Acts on gram-negative bacilli, but much higher concentrations required. Penicillin Binding Proteins (PBPs): β-lactam antibiotics have a complex mechanism of action. They acylate several bacterial proteins
termed penicillin binding proteins (PBPs). PBPs include but are NOT LIMITED to transpeptidase enzymes (D-alanine carboxypeptidase,
endopeptidase). Penicillins inhibit these enzymes by an irreversible covalent interaction. Particular β-lactam antibiotics bind to distinct PBPs. Binding is NOT uniform. The antimicrobial spectrum of action of certain β-lactam antibiotics is distinctive and related to their binding to some but
not all PBPs. Autolytic Activity. The presence of endogenous autolytic enzymes (murine hydrolases) is required for
bactericidal lytic effect. β-lactams “trigger” this activity by depressing the natural inhibitory action of
autolysins. Kinetics of penicillin bactericidal action. Penicillin may also activate autolytic action of enzymes.
121
Relevance of PBPs to clinical use
Effect persists when drug is gone as penicillin exerts a persistent injurious action due to its
covalent binding to bacterial proteins. Maximal killing rate is function of the growth rate of the organism
122
Resistance to penicillin
Production of penicillinase enzyme via a plasmid. Production of the β-lactamase is induced in the
presence of penicillin. May be transmitted to sensitive organisms by bacteriophages (transduction). Major problem with staphylococcus. NOTE: β lactamase is the generic term for enzymes that hydrolyze β-lactams, includes penicillinases and cephalosporinases. Alterations in penicillin-binding proteins. Responsible for methicillin resistance in staphylococci (MRSA) and penicillin resistance in pneumococci. Inability to penetrate into the bacterial cell. E.g., penicillin G can’t enter many gram-negative bacteria (e.g., Pseudomonas) due to presence of outer membrane. “Escape or Persisters. Metabolically inactive organisms or “L” forms can survive in a hypertonic environment like the kidney.
123
Absorption of penicillin
Penicillins are moderately strong acids. Highly water-soluble. Acid-lability impairs oral absorption of many types of penicillin (penicillin G [about 20%], methicillin, carbenicillin, ticarcillin). Thus, optimal absorption from an empty stomach (1 hr ac or 2 hrs pc). Oral doses must be much higher than parenteral doses for penicillin G. Chemical modification of R-group improves absorption by increasing acid stability (amoxicillin, penicillin V). Rapidly absorbed from IM parenteral sites. Use of insoluble salts to reduce absorption and extend duration of action. For example, procaine penicillin G and benzathine penicillin G.
124
Distribution of penicillin
Distribute throughout body. Penetrate into tissues poorly (largely ionized at physiological pH).
Highest concentration in liver, kidney, skin. Variable binding to plasma proteins. Can enter inflamed tissues or membranes (CSF, joint, eye) more readily than normal.
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Metabolism – Excretion of penicillin
Most penicillins excreted as active drug via the kidney (t1/2 < 1 hr).
90% by tubular secretion. Blocked by probenecid (can prolong duration of activity). Metabolism often increases to compensate in cases of renal failure (oxacillin-type). Excreted in breast milk (consider risk / benefit in use).
126
Individual penicillins
includes prototype, penicillinase- resistant, extended spectrum and β-Lactamase Inhibitors.
127
Prototype Penicillins
Relatively narrow spectrum of antimicrobial activity. Includes penicillin G and acid resistant penicillin.
128
Penicillin G
(Benzyl penicillin) [Pen G Benzathine: Bicillin, Permapen, Pen G Procaine: Wycillin,
Crysticillin]. The prototypical penicillin. Powerful and inexpensive. Previously penicillin of choice in most circumstances, but use today limited to hospitalized patients with serious infections given via the parenteral route. However, hydrolyzed by acid and penicillinase enzyme. About 30-50% bound to plasma protein.
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Acid resistant penicillin
Penicillin V (Phenoxymethyl penicillin) [Pen Vee K, V-Cillin]. Better absorbed than penicillin G, but still incompletely absorbed. Many prefer Penicillin V for oral therapy because of higher reliability of absorption. Antimicrobial efficacy generally less than penicillin G but still suitable for many mild-to-moderate infections.
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Penicillinase-Resistant Penicillins
In general order of efficacy (greatest first, but efficacy will vary depending on the particular organism): Methicillin [obsolete] > Nafcillin [Unipen (parenteral, erratic oral)] > Oxacillin [Prostaphlin, Bactocill (oral)] > Cloxacillin [Tegopen(oral)] and Dicloxacillin [Dynapen, Veracillin (oral)]. Considerably less potent against Penicillin G-sensitive organisms. Not substitutes for penicillin G, except when penicillinase-producing organisms are encountered (but emergence of MRSA has greatly limited current clinical use). Variable protein binding, high with oxacillin, cloxacillin and dicloxacillin. NOTE: Acid resistance varies among the penicillinase resistant penicillins as noted above. These penicillins are less susceptible to β-lactamase than are the cephalosporins. As a group the isoxazole penicillins are eliminated by both renal and hepatic routes improving their safety profile in patients with renal insufficiency. All are relatively narrow spectrum agents: Gram-positive, gram-negative cocci.
131
Extended Spectrum Penicillins
Increased hydrophilicity [due to presence of amino (NH2) or carboxyl (COOH) groups] allowing penetration through porins of outer membrane of gram-negative organisms.
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Ampicillin [Omnipen] and amoxicillin [Amoxil]
they are extended spectrum penicillin. Possess significant additional activity against gram-negative bacilli. Acid resistant, but NOT resistant to penicillinase. Amoxicillin is more completely absorbed after oral administration and food interferes less with its absorption (advantage of less frequent dosing and less diarrhea). Not resistant to penicillinase, but both can be given with β- lactamase inhibitors to further extend their antimicrobial spectrum.
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Anti-pseudomonal penicillins
(not resistant to penicillinase). Ticarcillin [Ticar] and Piperacillin [Pipracil]: Must be given parenterally. Effective against Pseudomonas aeruginosa and enterococci (often combined with
aminoglycosides). Also useful in anaerobic infections caused by Bacteroides fragilis.
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β-Lactamase Inhibitors
Clavulanic Acid, Sulbactam, Tazobactam. Another pharmacologic approach to combat penicillin resistance. Resemble β-lactam molecules but have weak or no antibiotic activity. They act as potent, irreversible inhibitors of β-lactamase. Most active against plasmid-encoded β-lactamases. The β-lactamase inhibitor will extend the antibacterial spectrum of the accompanying penicillin only if bacterial resistance is due to β-lactamase destruction and the inhibitor is active against that particular β- lactamase. Clavulanic acid combined with amoxicillin (Augmentin, oral) and ticarcillin (Timentin, parenteral), sulbactam combined with ampicillin (Unasyn, parenteral), and tazobactam combined with piperacillin (Zosyn, parenteral).
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Penicillin for gram positive cocci
Streptococci causes pharyngitis, pneumonia (CAP), sinusitis, otitis media, rheumatic fever, necrotizing fasciitis [Pen G, Pen V, Amoxicillin].
Enterococci causes bacteremia [Pen G, Ampicillin plus AG], urinary tract infection. [Ampicillin]. Staphylococcus aureus causes localized cutaneous infection, bacteremia, device-associated infections, pneumonia [MSSA: Oxacillin, MRSA: NO Penicillins].
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Gram-negative cocci with penicillin
Neisseria gonorrhea causes gonorrhea [Pen G, but high levels of resistance].
Neisseria meningititis causes meningitis [Pen G].
Moraxella catarrhalis causes otitis media, community-acquired pneumonia [Amoxicillin - Clavulanate].
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Gram-positive bacilli with penicillin
Bacillus anthracis causes anthrax [PenG].
| Cornyebacterium diphtheria causes diphtheria [PenG].
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Gram-negative bacilli with penicillin
[Ampicillin, Amoxicillin +/- Clavulanate]
H. influenzae causes meningitis, otitis media, sinusitis, pneumonia (CAP).
E. coli causes urinary tract infections, intra-abdominal infections, diarrhea, hemorrhagic colitis.
Klebsiella causes urinary tract infections, intra-abdominal infections.
H. pylori causes peptic ulcer disease.
Salmonella species causes gastroenteritis, typhoid fever.
Pseudomonas aeruginosa causes opportunistic infections in any organ or tissues [Amoxicillin-Clavulanate, Piperacillin - Tazobactam, Ticarcillin - Clavulanate].
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Anaerobes
| with penicillin
```
Clostridium perfringens (gram + rod) causes gas gangrene, food poisoning [Pen G].
Bacteroides fragilis (gram – rod) causes intraabdominal and brain abscess [Piperacillin - Tazobactam, Ticarcillin - Clavulanate].
```
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Spirochetes with penicillin
Treponema pallidum causes syphilis [PenG]. Borrelia burgdorferi causes Lymedisease (early) [Amoxicillin].
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Toxicity and Adverse Reactions with penicillin
Virtually non-toxic, except for hypersensitivity reactions.
142
Hypersensitivity Reactions with penicillin
Penicillin presumably functions as a hapten, combining with protein to form a complete antigen. Most reactions related to 6-aminopenicillanic acid portion of molecule; thus cross- sensitization occurs among penicillins. Classified by time to develop and immunologic type. IMPORTANT to note: 10% of patients report a penicillin allergy. Only 10-20% of these patients actually experience a reaction of some kind on exposure to a penicillin (1-2% overall). Classification of penicillin reactions are Type I, II, III
and IV. Other reactions are idiopathic, exact immunological mechanisms unknown. Most common reaction (1-4%) is a maculopapular or morbilliform rash that is generally mild and reversible.
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Type I Reactions with penicillin
Very rare (0.05%), but potentially life threatening. Mediated by IgE (mast cells) antibodies: Urticaria, angioedema, respiratory obstruction, vascular collapse. Onset ranges from a few minutes to about 30 minutes.
144
Type II Reactions with penicillin
Rare, due to cytotoxic antibodies of the IgG or IgM class. Complement-dependent cell destruction; e.g. hemolytic anemia.
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Type III Reactions with penicillin
Delayed allergic reactions (> 72 hours). Formation of IgG or IgM immune complexes with penicillin that act as antigens and can activate complement and lodge in tissue. Skin rashes, serum sickness, arthralgias, allergic vasculitis (inflammation of vascular bed).
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Type IV Reactions with penicillin
Cell mediated allergy, delayed reactions. Mediated by T-lymphocytes. Skin eruptions, thrombocytopenia.
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Other Toxicities than hypersensitivity with penicillin
Rarely, encephalopathy or seizures can occur with intrathecal penicillin or massive parenteral doses of
penicillin, especially if associated renal insufficiency. Beware of potassium or sodium intoxication when massive doses of salts and penicillin are administered
parenterally. Especially a problem if renal insufficiency or CHF. Jarisch-Herxheimer reaction during treatment of syphilis (via spirochetal antigens).
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Vancomycin
[also synthetic lipoglycopeptide derivative – Telavancin (Vibativ)
149
Structure / Mechanism of Action of Vancomycin
Tricyclic glycopeptide acts by inhibiting cell wall synthesis at site
different from penicillin (blocks linear polymerization, Stage 2 of cell wall synthesis).
150
Pharmacokinetic Properties of Vancomycin
Poor oral absorption, administered IV , except for GI tract indications (e.g., Clostridium, but
metronidazole is now preferred agent due to resistance concerns and expense of vancomycin). Excretion mainly through kidneys, in renal failure half-life extended to 6-10 days.
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Antimicrobial Spectrum / Clinical Uses
Use reserved for situations when less toxic agents are ineffective or not tolerated (e.g., penicillin allergy).
152
Gram positive cocci
with Vancomycin
Methicillin Resistant Staphylococcus Aureus (MRSA) causes severe skin and soft tissue infections. Staphylococci and streptococci causes meningitis, pneumonia, endocarditis, sepsis.
Enterococci (ampicillin resistant) causes bacteremia, endocarditis.
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Anaerobes with vancomycin
Clostridium difficile causes pseudomembranous colitis [vancomycin is used orally - parenteral route does not deliver to GI tract]; metronidazole 1st choice if NOT severe infection
154
Adverse Reactions with vancomycin
Chills-fever-skin rash (infusion-related), ototoxicity most severe (pretreat with acetaminophen and diphenhydramine). More highly purified preparations now available display fewer adverse effects.
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Daptomycin (Cubicin).
A cyclic lipopeptide, “promoted” as a parenteral, once daily, more rapidly bactericidal alternative to vancomycin. Mechanism involves action at bacterial membrane and loss of intracellular ions leading to cell death. Active against methicillin and vancomycin resistant strains of staphylococci (MRSA and VRSA) and vancomycin resistant enterococci (VRE). Minor side effects (GI disorders, fever, HA, dizziness). Restricted use.
156
Cephalosporins structure
Similar to penicillin nucleus, containing 7-aminocephalosporanic acid nucleus [A] instead of 6- aminopenicillanic acid structure. Cephalosporins are β-lactam antibiotics [B]. Substitutions can be made at the R1 and R2 positions to produce the various cephalosporins.
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Cephalosporins
Mechanisms of action, resistance, and pharmacology are similar to those of the penicillins. In general, relative to penicillins (G and V), cephalosporins have: Broader spectrum of action vs gram-negative bacteria; Less susceptibility to penicillinase (a β-lactamase) but cephalosporinases are emerging; Less cross-reactivity in penicillin sensitive patients (on the order of 1%)
158
Absorption of Cephalosporins
Several agents can be given orally (acid-stable) while others must be administered
parenterally (IV-IM)
159
Distribution of Cephalosporins
Cephalosporins penetrate well into most tissues and fluids (including placenta) except
brain and CSF. Major pharmacokinetics feature of 3rd generation agentsàpenetration into CSF
160
Metabolism – Excretion of Cephalosporins
Primarily excreted by kidneys, thus require dosage adjustment in renal
insufficiency (exception ceftriaxone)
161
Classifications of Cephalosporins
Cephalosporins are now classified into 5 generations. Breadth of activity against gram- negative organisms was the original basis for this classification but now considers resistance to inducible cephalosporinases and activity against methicillin-resistant Staph aureus (MRSA)
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First Generation of Cephalosporins
Cefazolin (Ancef), Cefadroxil (po-Duricef), Cephalexin (po-Keflex). Effective against many gram-positive cocci (except low activity against enterococci and methicillin-resistant staphylococci) commonly found on the skin, gram-negative cocci, and some gram-negative bacilli (Proteus, E. coli, Klebsiella). Rarely drugs of first choice.
Antibacterial spectrum like amoxicillin. More stable than penicillins to many beta-lactamases. Cefazolin is now the prototype for reasons of cost, lower toxicity, good penetration into most
tissues and lengthened t1/2 (90 min). Compared to penicillin G and later generation cephalosporins, the first generation
cephalosporins have greater activity against S. aureus (MSSA)
163
Second Generation of Cephalosporins
Cefuroxime (Zinacef, po-Ceftin), Cefotetan (Cefotan), Cefoxitin
(Mefoxin), Cefprozil (po-Cefzi), Cefaclor (po-Ceclor). Maintain gram positive coverage similar to 1st generation agents, but greater activity against
gram-negative bacteria (Hemophilus influenzae, M. catarrhalis, Neisseria species), especially
respiratory pathogens. Little or no activity against Pseudomonas species. Increased spectrum of activity is attributable to increased penetration of gram-negative
envelope and increased affinity for penicillin-binding proteins. Active against cefazolin (1st generation)-resistant strains of E.coli, Klebsiella, Proteus mirabilis. Active against anaerobes, including Bacteroides fragilis: – cefoxitin, cefotetan
164
Third Generation of Cephalosporins
: Cefdinir, (po-Omnicef), Cefotaxime (Claforan), Ceftriaxone (Rocephin), Cefotaxime (Claforan),Ceftazidime (Fortaz), Cefixime (po-Suprax), Cefpodoxime (po-Vantin). Maintain varying degrees of gram positive coverage, excellent activity against pneumococci (cefotaxime and ceftriaxone). Compared with 2nd generation agents, they have expanded gram negative coverage. More active against enteric gram-negative bacilli; also Enterobacter, Hemophilus, Neisseria.
165
Ceftazidime
has moderate antipseudomonal activity, but much less active against gram +
166
Fourth Generation of Cephalosporins
Cefepime (IV-Maxipime). Similar to 3rd generation agents, but more resistant to chromosomal and extended spectrum β- lactamases. Good activity against Pseudomonas species and S. pneumonia. Some anaerobic coverage, but not B. fragilis
167
“Fifth” (next) Generation of Cephalosporins
Ceftaroline (IV-Teflaro). Similar to 3rd generation (ceftriaxone) against gram negative with better gram-positive coverage including MRSA, S. pneumoniae, and E. faecalis. Does not cover Pseudomonas species and has limited anaerobic activity
168
Antimicrobial Spectrum First Generation of Cephalosporins.
Rarely drugs of choice despite low toxicity and broad spectrum of activity.
169
Gram positive cocci of first generation
with Cheaplosporin
Staphylococci (MSSA), Streptococci causes surgical prophylaxis, skin infections (non-MRSA) [Cephalexin (po), Cefazolin (IV)].
170
Gram negative bacilli
of first generation
with Cheaplosporin
Klebsiella causes pneumonia, urinary tract infections [Cephalexin] E. coli, Proteus causes urinary tract infections [Cephalexin]
171
Second Generation of Cephalosporins
Gram negative bacilli.
Resistant E. coli, Proteus, Klebsiella causes pneumonia, urinary tract infections
H. influenza causes meningitis [Cefuroxime (IV)], otitis media, sinusitis [Cefaclor (po)] M. catarrhalis causes otitis media, sinusitis [Cefaclor (po)]
172
Second Generation of Cephalosporins
Anaerobes
Bacteroides (gram – rod) causes peritonitis, diverticulitis [Cefoxitin]
173
Third Generation
of Cephalosporins Gram-positive cocci.
S. pneumoniae causes pneumonia, meningitis [Cefotaxime (especially useful in children, Ceftriaxone]
174
Third Generation
of Cephalosporins Gram negative cocci
N. gonorrheae causes gonorrhea; N. meningitidis causes meningitis [Ceftriaxone]
175
Third Generation
of Cephalosporins Gram negative bacilli
.
Pseudomonas aeruginosa causes urinary tract infections, pneumonia [Ceftazidime] H. influenza causes pneumonia, meningitis [Cefotaxime (especially useful in children)] E. coli causes urinary tract infection (complicated) [Cefotaxime, Ceftriaxone] Klebsiella causes urinary tract infection (complicated), pneumonia [Ceftriaxone] Unknown etiology causes sepsis [Ceftazidime]
176
Fourth Generation of Cephalosporins
[Cefepime]
177
Fourth Generation of Cephalosporins Gram-positive cocci
.
S. pneumoniae causes pneumonia
178
Fourth Generation of Cephalosporins Gram negative cocci.
M. catarrhalis causes pneumonia
179
Fourth Generation of Cephalosporins Gram negative bacilli.
Pseudomonas aeruginosa causes urinary tract infections, pneumonia H. influenza causes pneumonia
E. coli causes urinary tract infection (complicated) Klebsiella causes urinary tract infection (complicated), pneumonia Unknown etiology causes sepsis, neutropenic fevers
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Adverse Reactions - Toxicity of Cephalosporins
Generally well tolerated due to high selective toxicity. Allergy / Hypersensitivity: Can see anaphylaxis, skin rashes, nephritis, hemolytic anemia. Reactions are not as severe as with penicillins. Cross-hypersensitivity with penicillins exists (but less than 1%); increased risk with 1st generation agents.
NOTE: Should not be given to patients with history of immediate sensitivity to penicillin
[although 2nd-3rd-4th generation agents have been tried with certain precautions]. Nausea, vomiting, diarrhea; local irritation on injection possible (thrombophlebitis)
• Superinfection (including pseudomembranous colitis) with 2nd and 3rd generation agents (broader
spectrum agents). Action to suppress intestinal flora (decrease synthesis of Vit K) can intensify effect of oral anticoagulants
181
Carbapenems
Imipenem / Cilastatin (Primaxin), Meropenem (Merrem), Ertapenem (Invanz)
182
Mechanism of Action of Carbapenems
Structurally related to β-lactam antibiotics. Carbapenems are β-lactamase resistant, but imipenem is inactivated by renal dihydropeptidase (thus administered with cilastatin, an inhibitor of the enzyme). The carbapenems readily gain access to organisms where it is an inhibitor of bacterial cell wall synthesis. They interact with the penicillin-binding proteins (considered to be transpeptidases) responsible for cell wall elongation (stage 3 of cell wall synthesis).
183
Pharmacokinetics of Carbapenems
Must be given parenterally - IV/IM. Penetrates all tissues, including CSF. Imipenem is metabolized in the kidney by renal dipeptidase and then eliminated by the kidney. Meropenem and ertapenem are not sensitive to renal dipeptidase and are not given with cilastatin. Half-life increases in the presence of renal failure requiring dosage reduction. Ertapenem has the longest half-life and can be administered once daily.
184
Antibacterial Spectrum / Clinical Uses of Carbapenems
Generally held in reserve for treatment of infections resistant to multiple drugs. Carbapenems are active against both gram-positive and gram-negative aerobic and anaerobic bacteria including enterococci, but lack activity against MRSA and E. faecium. They exhibit bactericidal activity towards organisms resistant to the aminoglycosides and cephalosporins. β-lactamase producing organisms are also sensitive
185
Gram-negative bacilli with Carbapenems.
Pseudomonas aeruginosa, E. coli, Serratia, Citrobacter causes urinary tract infections, respiratory infections, bacteremias
186
Anaerobes with Carbapenems.
Bacteroides causes bacteremia, abscess (intra-abdominal, lung, brain) Clostridium perfringens (gram + rod) causesgas gangrene
187
Adverse Reactions with Carbapenems
Most common adverse effects are nausea / vomiting, diarrhea, skin rash (some cross-sensitivity with
penicillins). Seizures noted in small number of patients with higher doses (usually older patients with history of CNS
disease and renal insufficiency); seizure incidence lower with meropenem
188
Monobactams
Aztreonam (Azactam)
189
Mechanism of Action with Monobactams
Semisynthetic, monocyclic β-lactam ring, resistant to β-lactamases. Antibacterial activity due to interactions with certain of the penicillin-binding proteins. Only gram-negative aerobic bacteria are susceptible. Gram-positive and anaerobic pathogens lack sensitivity.
190
Pharmacokinetics with Monobactams
Not available orally, but well absorbed from intramuscular sites. Distributed to most tissues including CSF; higher levels in CSF if meninges inflamed. Elimination primarily via kidneys with t1/2 of 1.7 hrs.
191
Antimicrobial Spectrum / Clinical Uses with Monobactams:
Resembles aminoglycosides in activity (gram-negative bacilli), but much less toxic. Synergism with aminoglycosides against Pseudomonas aeruginosa. Has been used alone in the management of gram-negative respiratory, urinary, and bone and joint infections.
192
Adverse Effects with Monobactams
In general, very safe drug. Mild gastrointestinal upset, eosinophilia, skin rash, transient elevation of liver enzymes. Can be tolerated by penicillin-allergic patients.
193
Macrolides
Erythromycin [Erythrocin and others], Clarithromycin [Biaxin], Azithromycin [Zithromax]
194
Mechanism of Action of Macrolides
Inhibition of microbial protein synthesis. Binds to 50S ribosomal subunit blocking translocation of
peptidyl tRNA from acceptor site to donor site on ribosome and peptide bond formation, thus stopping protein synthesis.
Not actively transported, enters bacteria by passive diffusion. A weak base that is more active at alkaline pH. Bacteriostatic- may be bactericidal at higher concentrations for susceptible organisms. Selectively toxic because no binding occurs to mammalian 60S ribosome. Resistance occurs due to methylation of 50S ribosome, thus preventing erythromycin binding (can occur rapidly); also substrates for multi-drug efflux transporter (coded for by MDR gene).
Inactivation of drug does not occur with most organisms (some bacterial esterases may emerge).
195
Absorption of Macrolides
Can be absorbed from GI tract.
Erythromycin: Absorption varies depending on salt form. [Free base] destroyed by stomach acid, administered with enteric coating [Stearate, estolate (Ilosone), ethyl succinate (EES)] fairly acid-resistant and well absorbed. Some forms (stearate, base) show decreased absorption if food in stomach. Estolate appears more bioavailable in children than ethyl succinate.
Clarithromycin: Can be taken without regard to meals (food may improve absorption). Azithromycin: Should be taken on empty stomach (not with food)
196
Distribution of Macrolides
Distributed widely, except brain and CSF; transverses placenta and reaches the fetus. Azithromycin and clarithromycin reach higher concentrations in certain tissues than in plasma
(skin, lungs, tonsils, cervix, sputum); also accumulate in macrophages.
197
Metabolism / Excretion of Macrolides
Erythromycin is metabolized in liver and excreted in bile (avoid if liver disease present). Clarithromycin metabolized to active compound that is renally eliminated (may require adjustment in renal disease). Azithromycin NOT metabolized; high tissue penetration and
binding with subsequent slow release allows once-daily dosing. Excreted in breast milk. Use OK, but observe infant for candidal thrush, diarrhea.
198
Antimicrobial Spectrum / Clinical Uses of Macrolides in
| Gram positive cocci
(alternative in penicillin-allergic patients for sensitive organisms) Streptococci, pneumococci causes pneumonia, pharyngitis [All]
199
Antimicrobial Spectrum / Clinical Uses of Macrolides in
| Gram-negative cocci.
Moraxella catarrhalis causes otitis media, community-acquired pneumonia [All]
200
Antimicrobial Spectrum / Clinical Uses of Macrolides in
| Gram positive bacilli
Corynebacterium diptheriae causes diphtheria [Erythromycin]
201
Antimicrobial Spectrum / Clinical Uses of Macrolides in
| Gram-negative bacilli
Bordetella pertussis causes whooping cough [All].
H. influenza causes upper respiratory infections, bronchitis [Azithromycin, Clarithromycin]. H. pylori causes peptic ulcer disease [Clarithromycin (plus PPI or H2 antagonist)]. Legionella causes community-acquired pneumonia [Azithromycin]. Mycobacterium avium causes pneumonia [Clarithromycin, Azithromycin].
202
Antimicrobial Spectrum / Clinical Uses of Macrolides in
| Atypical organisms
Chlamydia causes trachoma, community-acquired pneumonia, urethitis [Azithromycin] Mycoplasmapneumoniae causes community-acquiredpneumonia [All]
203
GI Disturbances of Macrolides
Nausea, vomiting, diarrhea, anorexia; due to direct stimulation of gut motility by
erythromycin, less with azithromycin and clarithromycin.
204
Hepatotoxicity of Macrolides
Reversible acute cholestatic hepatitis (estolate salt).
205
Prolongs QT effect of Macrolides
interval causes ventricular arrhythmias. Use caution with other QT prolonging drugs.
206
Drug Interactions of Macrolides
Erythromycin metabolites and clarithromycin can inhibit cytochrome P450
enzymes [NOT azithromycin] and increase plasma levels and potential toxicity of numerous drugs including: theophylline, warfarin, methylprednisolone, cyclosporine, SSRIs, benzodiazepines.
207
Telithromycin (Ketek)
a ketolide antibiotic, derived from erythromycin. Structural features allow for tighter binding to 50S ribosome, along with increased acid stability and less affinity for drug efflux transporter (effective against macrolide-resistant organisms). Dosed once-a-day and is a substrate and inhibitor of CYP3A4. GI disturbances are most common side effect; visual difficulties (1-2%). Now linked to dozens of cases of severe liver injury or failure, including several deaths. Use ONLY for community-acquired pneumonia due to Strep pneumoniae that is resistant to other antibiotics.
208
Tetracyclines
Tetracycline [Achromycin V], Doxycycline [Vibramycin], Minocycline [Minocil], Tigecycline [Tygacil]
209
Mechanism of Action of Tetracyclines
Bacteriostatic inhibition of bacterial protein synthesis. Specific binding to 30S ribosome (reversible) prevents access of aminoacyl tRNA to site on mRNA-
ribosome complex blocking addition of amino acids to peptide chain. Some selective toxicity because mammalian cells have an active efflux mechanism preventing
intracellular accumulation of drugs and lack an active transport for moving drug into cell. Resistance occurs due to: (1) changes in proteins that transport drug into cell or membrane insertion of proteins that actively transport tetracycline out of cell (plasmid can code for multiple drug resistance [TCNs / CHLOR / SULF / AG / MAC]), (2) ribosome protection by producing proteins that block
tetracycline binding. Tigecycline NOT affected by these mechanisms.
210
Absorption of Tetracyclines.
Most are adequately absorbed (tetracycline incomplete) (best given on empty stomach); presence (retention) in GI tract alters intestinal flora and contributes to superinfections. Minocycline, doxycycline highest bioavailability (95-100%). Absorption is impaired by milk products, Al+++, Ca++, Mg++, and Fe++ salts (formation of
insoluble salts that can’t be absorbed). Distribution of Tetracyclines: Varying degrees of protein binding (20-95%); penetration into most tissues/fluids is
excellent, including placental / fetal circulation
211
Metabolism / Excretion of Tetracyclines.
Concentrated in liver, secreted into bile (use with caution in impaired liver function); subject to enterohepatic recirculation (contributes to maintenance of plasma levels). Most excreted into urine, important exceptions are doxycycline and minocycline (thus, doxycycline is choice for patients with underlying renal disease) and breast milk. Tetracycline is short-acting (t1/2 of 6-8 hours); doxycycline and minocycline are long-acting (t1/2 of 16-18 hours).
212
Antimicrobial Spectrum / Clinical Uses of Tetracyclines
Broad spectrum agent but overuse has led to widespread resistance for many organisms.
213
Gram positive cocci with Tetracyclines.
```
(most organisms are resistant)
Staphylococcus aureus (methicillin-resistant-MRSA) causes localized cutaneous infection, pneumonia, food poisoning [Doxycycline, Tigecycline]
```
214
Gram-negative cocci with Tetracyclines.
(many organisms are resistant)
| Moraxella catarrhalis causes otitis media, community-acquired pneumonia [Doxycycline]
215
Gram positive bacillus with Tetracyclines.
Bacillus anthracis causes anthrax [Doxycycline]
216
Gram-negative bacilli with Tetracyclines
(many organisms are resistant)
H. influenza causes otitis media, community-acquired pneumonia [Doxycycline] H. pylori causes peptic ulcers. [Tetracycline]
Vibrio cholera causes cholera [Doxycycline]
217
Anaerobes with Tetracyclines
Propionobacterium acnes (gram+rod) causes acne [Minocycline,Doxycycline]
218
Atypical organisms with Tetracyclines.
Chlamydia causes trachoma, community-acquired pneumonia, urethitis [Doxycycline] Mycoplasma pneumoniae causes community-acquired pneumonia [Doxycycline] Richettsia causes Rocky Mountain spotted fever, Q fever [Doxycycline]
219
Spirochetes with Tetracyclines.
Borreliaburgdorferi causes Lymedisease (early) [Doxycycline]
220
Tigecyline
a new agent which could be useful in treatment of complicated infections with resistant organisms (MRSA, penicillin-resistant S. pneumoniae, VRE, and many gram negative organisms).
221
Teeth and bone with Tetracyclines
Temporary depression of bone growth, permanent discoloration of teeth if given
during tooth development [chelates to Ca++ in developing bone and teeth]. Avoid use during latter
half of pregnancy and in children under 8 years old (Pregnancy Risk Factor D).
222
GI disturbance with Tetracyclines
Nausea, vomiting, diarrhea common
223
Photosensitivity with Tetracyclines
Abnormal sunburn reaction.
224
Yeast (candidal) overgrowth with Tetracyclines
Disturbance of normal gut flora can lead to thrush and vaginitis.
225
Liver / kidney toxicities with Tetracyclines
Some reports, especially if pre-existing conditions.
226
Drug Interactions
| with Tetracyclines
Antacids / Iron Supplements (metal ions): decreases bioavailability by forming insoluble salts. Phenytoin / Barbiturates / Carbamazepine: Increased metabolism of doxycycline. Oral Anticoagulants: Increased anticoagulant effect
227
Clindamycin (Cleocin) Mechanism of Action.
Inhibition of protein synthesis by binding to 50S ribosome and preventing translocation of peptidyl
tRNA (possibly at erythromycin binding site) and peptide bond formation. Bacteriostatic, but can be bactericidal against certain organisms at higher concentrations.
228
Clindamycin (Cleocin) Pharmacokinetic Properties Absorption
90% of oral dose absorbed, not affected by presence of food. Distribution: Penetrates most tissues well, especially bone, but not well into CSF. Metabolism / Excretion: Metabolized by liver, then primarily biliary excretion, no dosage adjustment
required in renal failure; excreted in breast milk.
229
Antibacterial Spectrum / Clinical Uses of Clindamycin (Cleocin).
Treatment of severe anaerobic infections.
230
Gram positive cocci with Clindamycin (Cleocin).
Streptococci causes pneumonia, pharyngitis (alternative if penicillin allergic) Staphylococcus aureus (MSSA) causes osteomyelitis (alternative if penicillin allergic) Staphylococcus aureus (MRSA) causes localized cutaneous infection
231
Anaerobes with Clindamycin (Cleocin).
Clostridium perfringens (gram + rod) causes gas gangrene, food poisoning Bacteroides fragilis (gram – rod) causes intra abdominal and brain abscess Propionobacterium acnes (gram+rod) causes acne (topically)
232
```
Adverse Reactions
with Clindamycin (Cleocin).
```
Pseudomembranous colitis. Toxigenic Clostridium difficile selected out during treatment
(superinfection, 0.1-10%). Probably no worse than some broader spectrum agents (amoxicillin-
ampicillin, 2nd/3rd generation cephalosporins, fluoroquinolones). Commonly: Nausea, diarrhea (severe 2-20%), skin rashes. Rarely: Impaired liver function, neutropenia
233
Aminoglycosides
Streptomycin, Tobramycin, Gentamicin, Amikacin, Kanamycin, Neomycin
234
Aminoglycosides Structure:
Aminoglycosides have a hexose ring (streptidine or 2-deoxystreptamine) with various amino sugars attached by glycosidic linkages. They are water-soluble (distribute into extracellular fluid, but enter
cells poorly), stable in solution and more active at alkaline pH than at acid pH.
235
Mechanism of Action for Aminoglycosides.
Bacteriostatic at low concentration. Bactericidal at high concentrations (clinically is bactericidal). Actively transported into bacteria; requires O2. Not effective against anaerobic organisms. Presumably acts by inhibition of protein synthesis. Combines with 30S ribosome, altering the
interaction of mRNA with the subunit, producing inhibition of protein synthesis initiation, breakup
of polysomes, and misreading the code.
236
Mechanisms of Resistance for Aminoglycosides.
Microbial resistance to the aminoglycosides is mediated by mechanisms
that vary from organism-to-organism. Three mechanisms have been identified:
Chemical modifications of the antibiotic that impair ribosomal binding and further drug uptake. The amino and hydroxyl groups of the aminoglycosides are essential for antimicrobial activity. Plasmid mediated transfer of the ability to chemically modify the
aminoglycosides is of major importance for gram-negative pathogens. Mechanisms of lesser importance include decreased antibiotic penetration into the organism and changes in the ribosomal target site with decreased ability to bind drug.
237
Absorption of Aminoglycosides
Highly polar cationic compounds that are NOT significantly absorbed after oral administration. Rapid and complete absorption occurs after intramuscular administration giving peak plasma concentrations in 30-90 minutes
238
Distribution of Aminoglycosides.
Distribution in body limited to extracellular fluid. Not concentrated in mammalian cells.
Binding to plasma proteins varies among the aminoglycosides (0-30%). Largely excluded from central nervous system (but can reach 20% of plasma levels if inflamed) and the eye. Selectively accumulates in the renal cortex and inner ear
predisposing these organs to toxicity.
239
Metabolism / Excretion of Aminoglycosides.
Not metabolized. Aminoglycosides are eliminated by excretion through the kidneys with the normal half-life of 2-3 hrs. Renal clearance is directly proportional to creatinine clearance. Dosage adjustment required if impaired renal function to avoid drug accumulation and toxicity. Once daily dosing: Aminoglycosides have concentration-dependent killing (increasing concentrations kill increasing proportion of bacteria at faster rate) and a postantibiotic effect (bactericidal effect persists beyond plasma half-life) which allows their administration in a single large dose vs 2-3 equally divided doses.
240
Pharmacokinetics of aminoglycosides
are subject to great interindividual variation even in patients with normal renal function. This fact together with the narrow therapeutic index of this class of antibiotics mandates monitoring of plasma drug concentration, and renal and eighth cranial nerve function. Blood is sampled 30 min after an IV infusion or 60 min after IM injection. Trough levels are obtained just before the next dose.
241
Antibacterial Spectrum / Clinical Uses of Aminoglycosides.
Declining use due to toxicity, being replaced by (or used in combination with) fluoroquinolones, antipseudomonal penicillins, and 3rd generation cephalosporins.
242
Gram-negative bacilli with Aminoglycosides
[Gentamicin, Tobramycin, Amikacin]
Pseudomonas aeruginosa causes infections in any organ or tissues (especially nosocomial)
E. coli, Klebsiella, Serratia, Proteus, Enterobacter (enteric gram negative) causesurinary tract infections, lower respiratory infections, bacteremias
presurgery bowel sterilization [Neomycin, oral], wound infection [Neomycin (topical)]. Mycobacterium tuberculosis causes tuberculosis [Streptomycin plus multiple antitubercular drugs]
243
Gram positive cocci with Aminoglycosides
Enterococci causes bacteremia, endocarditis, intraabdominal infections [Gentamicin plus a penicillin or vancomycin]
244
Adverse Reactions / Toxicity with Aminoglycosides.
VERY TOXIC Eighth nerve damage (often irreversible). The site of this effect likely involves sensory receptors of
the nerve. Concentrated in perilymph and affects cell membrane phosphatidyl inositol metabolism and appears to affect Ca++ fluxes. Auditory (0.5-12%): Tinnitus and high frequency hearing loss (outside normal speech). Vestibular (1-3%): Dizziness, nausea / vomiting, vertigo. Ethacrynic acid and furosemide potentiate the ototoxicity of aminoglycosides. Most likely to
occur in patients with impaired renal function (esp., the elderly). Dependent on dose and plasma level of aminoglycoside and duration of therapy. Peak levels predict efficacy; trough levels predict toxicity (time above threshold concentration).
245
Renal Toxicity with Aminoglycosides.
(usually reversible on discontinuation of drug). 25% of patients show mild impairment. Manifested by rising BUN and creatinine levels, proteinuria, oliguria, acute tubular necrosis,
followed by reduced glomerular filtration (resulting in further accumulation)
246
Sensitivity Reactions with Aminoglycosides
Contact dermatitis. Skin rashes. Bone marrow depression.
247
Neuromuscular blockade with Aminoglycosides
can lead to respiratory arrest, especially in susceptible persons (patients with
myasthenia gravis or patients on other neuromuscular blocking agents).
248
Drug-Drug Interactions with Aminoglycosides
Synergy: The in vitro and in vivo synergy between the β-lactams and aminoglycosides is likely the result of an enhanced aminoglycoside entry brought about by β-lactam damage to the cell wall. Inhibitory Effects: Irreversible binding of aminoglycosides to certain penicillins can result in inactivation of the aminoglycosides (has been demonstrated in vitro and in vivo). In vitro inactivation is avoided by administering each drug individually while in vivo inactivation may be minimized by the timing of drug administration (β-lactam first).
249
Chloramphenicol (Chloromycetin). Structure
First completely synthetic antibiotic of importance. Extremely lipid soluble.
250
Mechanism of Action of Chloramphenicol.
```
Potent inhibition of microbial protein synthesis. Reversible binding to 50S ribosome resulting in block of peptidyl transferase action and
incorporation of amino acids into newly formed peptides. Bacteriostatic (bactericidal against some Bacteroides, H. influenzae, N. meningitidis). Also inhibits mammalian mitochondrial protein synthesis in human bone marrow, thus
diminishing selective toxicity (mitochondria ribosome resembles bacterial 70S ribosome). Resistance due to mutant strains that are impermeable to drug (emerges slowly) or to inactivation by
bacterial enzymes (acetyl transferase, plasmid mediated)
```
251
Absorption of Chloramphenicol
Rapid and complete from GI tract; more variable in children
| Distribution: Widely distributed to all tissues and fluids, including CNS and CSF (50% of plasma)
252
Metabolism / Excretion of Chloramphenicol
Metabolized by glucuronidation (major pathway) or reduction. In fetus and neonate immature liver cannot conjugate, thus toxicity. Dosage reduction required in hepatic failure. Excreted in breast milk, thus avoid in pregnancy and while breast-feeding
253
Antimicrobial Spectrum / Clinical Uses of Chloramphenicol.
Broad spectrum agent, but potential toxicity limits use in US. Should be used only for severe infections when less hazardous drugs are ineffective or patient allergic.
254
Gram negative cocci with Chloramphenicol
N. meningitides causes meningitis
255
Gram negative bacilli with Chloramphenicol
H. influenza causes meningitis Salmonella typhi causes typhoid fever
256
Anaerobes
| with Chloramphenicol.
Bacteroides causes brain abscess, intraabdominal sepsis
257
Atypical organisms with Chloramphenicol
Rickettsia causes Rocky Mountain spotted fever, Q fever
258
Bone marrow toxicity with Chloramphenicol
Avoid if leukopenias, anemia, thrombocytopenia. Dose-related bone marrow depression (reversible). Important to monitor blood levels and perform complete blood and platelet count every 2-3 days. Aplastic anemia (1 in 30,000, not dose-related). Rare, but usually fatal; can appear weeks to months after therapy stopped
259
Gray baby syndrome
| with Chloramphenicol.
Immature hepatic function (reduced phase II glucuronidation activity) coupled with inefficient
renal function leads to toxic accumulations. Appears 2-9 days after therapy initiated. Vomiting, abnormal respiration, cyanosis leading (in
24 hrs) to vasomotor collapse and ashen-gray color development causes 40% mortality
260
Adverse reactions with Chloramphenicol.
GI disturbances: Nausea, vomiting, diarrhea. Oral or vaginal candidiasis (superinfection via alteration of normal flora). Drug Interactions: Inhibits metabolism of phenytoin, oral anticoagulants, and 1st generation oral
hypoglycemic agents
261
Linezolid (Zyvox).
First member of oxazolidinone class of antibiotics Mechanism of Action of Linezolid. Bacteriostatic. Binds to 50S ribosome (23S portion) at different site than other agents, thus no cross- resistance observed with other protein synthesis inhibitors. Inhibits early phase of protein synthesis (formation of 70S ribosome complex) with no effect on peptidyl transferase or termination reaction of protein synthesis.
262
Pharmacokinetics of Linezolin.
Oral bioavailability is 100%. Can take without regards to meals. IV preparations are available. Readily distributed to well-perfused tissues. Primarily metabolized by nonenzymatic oxidation (neither a substrate nor inhibitor of cytochrome
P450 enzymes). Also, some renal excretion of parent drug, but no dosage adjustments required for either renal or mild-moderate hepatic impairment. Half-life: 4-6 hours.
263
Antimicrobial Spectrum / Clinical Uses with Linezolid
Should be held in reserve for life-threatening infections.
264
Gram positive cocci
| Linezolid
Enterococci (includingVRE) causes bacteremia, endocarditis Staphylococci (including MRSA) causes complicated skin infections. Streptococcic causes pneumonia (community-acquired and nosocomial)
265
Adverse Effects of Linezolid.
Well tolerated, minor side effects (include diarrhea, headache, and nausea [1-10%]). However, thrombocytopenia or significant reduction in platelet count has occurred (2.4% incidence).
266
Important drug interaction
Reversibly and non-selectively inhibits MAO. A hypertensive response may occur with administration of sympathomimetic agents or foods high in tyramine or serotonin syndrome if given with SSRIs (serotonin selective reuptake inhibitors used in depression)
267
Quinupristin / Dalfopristin (Synercid).
First member of streptogramin class of antibiotics.
268
Mechanism of Action with Quinupristin / Dalfopristin.
Quinupristin and dalfopristin are combined in a 30:70 ratio in which they produce a sequential and
synergistic inhibition of bacterial protein synthesis. Quinupristin binds to 50S ribosome (same site as macrolides) and inhibits peptide elongation, leading
to early termination of protein synthesis. Dalfopristin binds at nearby site on 50s ribosome, inducing conformational change that both enhances quinupristin binding and directly interferes with peptide chain formation.
269
Pharmacokinetics with Quinupristin / Dalfopristin.
Must be administered intravenously where it can be irritating to veins. Biotransformation primarily via hepatic conjugation reactions with subsequent elimination via the
feces (biliary excretion)
270
Antimicrobial Spectrum / Clinical Uses with Quinupristin / Dalfopristin.
Should be held in reserve for life-threatening infections.
271
Gram positive cocci with Quinupristin / Dalfopristin.
Enterococci (includingVRE) causes bacteremia, endocarditis Staphylococci (including MRSA) causes complicated skin infections Streptococci causes pneumonia (community-acquired and nosocomial)
272
Adverse Effects with Quinupristin / Dalfopristin.
Common effects include infusion site irritation, arthalgia/myalgia, nausea, diarrhea, skin rashes
273
Important drug interaction with Quinupristin / Dalfopristin
Inhibits cytochrome 3A4 and may lead to increased plasma levels of 3A4 substrates such as: benzodiazepines cisapride, calcium channel blockers, carbamazepine,
cyclosporine, HMG CoA reductase inhibitors, and HIV protease inhibitors.
274
Fluoroquinolones
Norfloxacin [Noroxin], Ciprofloxacin [Cipro], Levofloxacin (levo isomer of ofloxacin) [Levoquin], Gemifloxacin [Factive], Moxifloxacin [Avelox]
275
Mechanism of Action of Fluoroquinolones
The primary molecular target is bacterial DNA gyrase and topoisomerase IV. DNA gyrase facilitates
unwinding of DNA strands and is required for normal DNA replication and transcription, and for some aspects of DNA repair and recombination. Inhibition of these actions by quinolones is rapidly bactericidal (99.9% lethal within 2 hr). Resistance emerges in about 1 in 107-109 organisms - likely due to one or more point mutations in binding site on DNA gyrase or possibly changes in drug permeability into organism. Plasmid-mediated resistance to fluoroquinolones has been increasing in U.S. hospitals. One type involves Qnr proteins that protect DNA gyrase from fluoroquinolones, while the other type codes for a variant of an aminoglycoside acteyltransferase that modifies ciprofloxacin.
276
Pharmacokinetic Properties of Fluoroquinolones.
```
Absorption: Well absorbed orally; levofloxacin and ciprofloxacin are also available parenterally. Distribution: Good penetration into most tissues including lung, bone, muscle, fat, prostate, and
reproductive organs (preliminary indication of CNS penetration if meninges inflamed); high urinary levels. Large volumes of distribution with minimal protein binding (< 25%). Metabolism / Excretion: ∼20% metabolism, but primarily excreted by kidney (t1/2’s prolonged in renal failure for some agents [ciprofloxacin, levofloxacin, NOT moxifloxacin]).
```
277
Gram positive cocci of Fluoroquinolones.
[Levofloxacin, Moxifloxacin, Gemifloxacin, i.e., “respiratory” quinolones] Strep. pneumoniae causes pneumonia, chronic bronchitis, acute sinusitis
Staph. aureus (MSSA) causes skin / skin structure infections
278
Gram-negative cocci with Fluoroquinolones.
[Levofloxacin] Moraxella catarrhalis causes sinusitis, bronchitis
279
Gram positive bacillus with Fluoroquinolones.
[Ciprofloxacin] Bacillus anthracis causes anthrax
280
Gram-negative bacilli with Fluoroquinolones.
[Ciprofloxacin, Levofloxacin, unless indicated]
H. influenzae causes lower respiratory tract infection, pneumonia [Moxifloxacin, Levofloxacin] Pseudomonas aeruginosa, E. coli, Klebsiella, Serratia, Proteus, Enterobacter (enteric gram negative) causes uncomplicated (generally no advantage over TMP/SMX) and complicated urinary tract infections Shigella, Salmonella causes Traveler’s diarrhea
281
Anaerobes with Fluoroquinolones.
[Moxifloxacin, Gemifloxacin]. Atypical organisms [Moxifloxacin, Gemifloxacin] Chlamydia causes trachoma, community-acquired pneumonia, urethitis Mycoplasma pneumoniae causes community-acquired pneumonia Richettsia causes Rocky Mountain spotted fever, Q fever
282
Adverse Reactions of Fluoroquinolones.
Overall very well tolerated. GI (5-10%): Nausea, vomiting, diarrhea (C. difficile associated). CNS: Dizziness, headache, insomnia, stimulation (seizures possible, but rare). Black Box Warning: Increased risk of tendon rupture (3-4-fold, but still rare, 1:10,000). Potential for
arthropathies limits use in pregnancy and children < 18 y/o. Rare: QT prolongation, glucose abnormalities with gatifloxacin, skin rashes
283
Theophylline: Increased toxicity due to
Drug Interactions of Fluoroquinolones.fluoroquinolone (esp. ciprofloxacin) inhibition of metabolism. Not seen with later generation agents.
Antacids (containing Mg++, Al+++, Ca++ ions) reduce oral absorption of ciprofloxacin. Must space antacid dosing quinolone administration.
284
Nitrofurantoin (Macrobid, Macrodantin).
Most commonly used agent of a group of drugs known as urinary tract antiseptics. These drugs cannot be used for treatment of systemic infections because effective plasma concentrations cannot be obtained with safe doses. However, because they concentrate in renal tubules, they can be given orally to treat urinary tract infections (i.e., non-systemic).
285
Mechanism of Action of Nitrofurantoin.
Reduced by bacterial enzymes to intermediates that damage bacterial DNA, producing a concentration dependent bacteriostatic or bactericidal effect. Selectively toxic because mammalian enzymes don’t reduce nitrofurantoin as rapidly.
286
Pharmacokinetics of Nitrofurantoin.
Rapid and complete absorption from GI tract; macrocrystalline form is absorbed more slowly. Nitrofurantoin is excreted into urine by both filtration and secretion with a half-life of 20-60 minutes. Rate is linearly related to renal function (creatinine clearance), so renal impairment can decrease drug efficacy and increase systemic toxicity and use is contraindicated. Colors the urine brown. Administered two (Macrobid) to four (Macrodantin) times daily
287
Adverse Reactions – Toxicity with Nitrofurantoin.
Most common side effects occur in the GI tract (anorexia, nausea, vomiting, diarrhea); macrocrystalline
forms are better tolerated. Occasional hypersensitivity reactions (hemolytic anemia, hepatocellular damage); neuropathies with
chronic use. Category B in pregnancy, but recent data suggest possible association with birth defects. Weigh
benefit-risk for use in pregnant patients
288
Antimicrobial Spectrum / Clinical Uses
of Nitrofurantoin.
active against many strains of E. coli and enterococci, but most species of Proteus and Pseudomonas are resistant, making nitrofurantoin a second line agent for treatment of urinary tract infections. HOWEVER, resistance of E. coli to TMP/SMX is increasing and nitrofurantoin, in addition to fluoroquinolones, can be considered for treatment of UTI in those areas with resistance > 20%. Also consider for use in uncomplicated UTIs in patients who are allergic to sulfa drugs
289
Metronidazole
(Flagyl) [also Tinidazole (Tindamax)] Mechanism of Action. Metronidazole is a prodrug that is transformed to a highly reactive nitro radical anion in susceptible
organisms (i.e., protozoa and anaerobic bacteria with a sufficiently negative redox potential). The anion kills these organisms by radical-mediated mechanisms that target DNA (strand breaks and / or inhibition of DNA replication. This is a bactericidal mechanism of action. Resistance in bacteria has been associated with expression of a nitroimidazole reductase that stops formation of the nitroso group responsible for bacterial killing.
290
Pharmacokinetics of Metronidazole.
Bioavailability is 80%. Absorption not affected by food (but time to peak delayed). Good distribution (including CSF and bone). Primarily hepatic metabolism, adjust dosage if liver impairment. Drug interactions possible with
inhibitors and inducers of CYP450. Breast feeding, should cease until 24-48 hrs after therapy (carcinogenic in animals)
291
Anaerobes with Metronidazole.
Clostridium perfringens (gram + rod) causes gas gangrene, food poisoning Clostridium difficle (gram + rod) causes diarrhea, pseudomembranous colitis Bacteroides fragilis (gram – rod) causes intraabdominal and brain abscess Helicobacter pylori (gram – rod, microaerophilic) causes gastritis, peptic ulcers
Protozoa
Trichomonas vaginalis causes trichomoniasis Entamoeba histolytica causes amebiasis Giardia lamblia causes giardiasis
292
Adverse Drug Reactions of Metronidazole.
Rarely severe enough to discontinue therapy. Most common are nausea, headache, dry mouth, metallic taste. Occasionally vomiting, diarrhea, and abdominal distress. Candidal superinfections (furry tongue, glossitis, and stomatitis). Inhibits aldehyde dehydrogenase and can produce an Antabuse-like effect (abdominal distress, vomiting, distress, headache) if alcohol consumed during or within 3 days of metronidazole therapy. Risk of severe reaction is low. Category B in pregnancy, but weigh benefit-risk. Conflicting evidence regarding teratogenicity in animals: Has been taken at all stages of pregnancy without adverse effects, but use during 1st trimester is not advised.
293
Mechanism of Action of Metronidazole.
Folic acid is required for synthesis of thymidine, purines, DNA, and certain amino acids. Bacterial cells cannot acquire folic acid from environment and must synthesize it from PABA. Sulfonamides are analogs of PABA that competitively inhibit dihydropteroate synthetase. They are bacteriostatic “antimetabolites” with a delayed onset of action (existing stores of folate must be depleted). NOTE: Humans can transport folic acid from the diet into cells and do not require the above synthetic pathway, accounting for the high level of selective toxicity of sulfonamides. However, inhibitors of dihydrofolic acid reductase have increased potential for human toxicity. Action noncompetitively antagonized by methionine, purines, and thymine (bypass of step requiring
folic acid). An example of resistance due to “escape”.
294
Bacterial Resistance with Metronidazole:
Resistance to sulfonamides is widespread and persistent in vivo (meningococci,
gonococci, β-hemolytic streptococci, enteric gram-negative rods). Acquired: Increased production of PABA or altered DHPS (decreased binding affinity for
sulfonamides): occurs in slow, stepwise fashion, usually does not emerge during therapy). Escape: The bacteria obtain end products methionine, homocysteine, serine, purines, thymine
from pus. Underlines importance of surgical drainage when indicated. Natural: Organisms with no folic acid requirement are not susceptible
295
Metronidazole Synergism with trimethoprim (TMP)
Trimethoprim inhibits dihydrofolate reductase (DHFR). Combination is bactericidal. A sulfonamide plus trimethoprim acts on two sequential enzymatic processes involved in tetrahydrofolic acid (FAH4) biosynthesis exerting a significant synergistic effect. Trimethoprim is NOT a sulfonamide. It is 5-20x more potent than sulfonamides. Effect of trimethoprim is rapid because FAH4 is depleted rapidly. (FAH2 form not useful to bacteria.) Trimethoprim used in combination with sulfamethoxazole (aka TMP/SMX, [Bactrim or
Septra]). Sulfamethoxazole has similar pharmacokinetics - 20:1 ratio is optimal for synergy. Resistance occurs due to altered dihydrofolate reductase. Pyrimethamine inhibits protozoal DHFR (useful in malarial infections) while methotrexate
inhibits mammalian DHFR (useful in certain neoplasms)
296
Absorption of Metronidazole.
Sulfonamides are weak acids that are generally well absorbed from GI tract (70-100%), peak levels within 2-3 hrs. Exception is those sulfonamides designed for local effects. Best to take on empty stomach, but with plenty of fluids.
297
Distribution of Metronidazole.
Widely distributed in body water including pleural, ocular, synovial fluids and CSF. Cross placenta and enter fetal circulation. Variable solubility, increasing with pH. Variable protein binding. Longer acting agents more completely bound to plasma proteins. Protein-binding displacement of bilirubin (can predispose to kernicterus in neonates) and protein-bound drugs is source of drug-drug interactions.
298
Metabolism / Excretion of Metronidazole.
Major metabolic pathway is N-acetylation to inactive compound (free amine group required for
sulfonamide activity). These metabolites can be toxic because of low solubility. Excreted unchanged by kidney. Rate of excretion in urine increases with pH. Decrease dose if
significant renal failure is present. Also excreted in breast milk.
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Antimicrobial Spectrum / Clinical Uses of Metronidazole.
Agents show quantitative rather than qualitative differences. Sulfonamides are BROAD SPECTRUM antibiotics: Gram positive / gram negative / chlamydia / protozoa (toxoplasma). Seldom used as single drug due to development of resistant organisms and to discovery of less toxic antibiotics. However, sulfamethoxazole (SMX) is commonly used with dihydrofolate reductase inhibitor trimethoprim (TMP] as TMP/SMX (Bactrim, Septra]).
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Gram positive cocci with Metronidazole.
Staph. aureus (community-acquired MRSA causes skin / skin structure infections [TMP/SMX]. Staphaureus causes conjunctivitis [Sulfacetamide]
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Gram-negative cocci with Metronidazole.
[TMP/SMX] Moraxella catarrhalis causes otitis media
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Gram-negative bacilli
with Metronidazole.
Klebsiella, Proteus, Enterobacter causes uncomplicated urinary tract infections [TMP/SMX]. Shigella causes diarrhea [TMP/SMX, preferred over fluoroquinolones in children]. Pseudomonasaeruginosa causes burninfections [topicalsilversulfadiazine]
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Atypical organisms with Metronidazole.
[TMP/SMX].
| Chlamydia causes trachoma, community-acquired pneumonia, urethitis
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Other infections with Metronidazole.
Pneumocystiscarinii causes pneumonia [TMP/SMX].
Nocardia causes nocardiosis (pulmonary infection) [Sulfisoxazole, sulfadiazine] Plasmodium(resistant) causes malaria [Sulfadoxine+Pyrimethamine] Ulcerative colitis [Sulfasalazine]
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Adverse Reactions / Toxicity with Metronidazole.
(generally safe, overall incidence 5%, higher with longer-acting agents). Sensitization reactions. Fever, rashes (~3% of patients) most common (sulfamethoxazole, sulfisoxazole). Rarely, agranulocytosis, aplastic anemia, serum sickness, anaphylaxis . Stevens-Johnson
syndrome can be fatal. Renal damage: Crystalluria in AIDS patients, minimize by adequate fluid intake. Rarely a problem
today with the most commonly used sulfonamide – sulfamethoxazole. Hemolytic anemia in patients with severe glucose-6-phosphate dehydrogenase deficiency in RBCs. Miscellaneous: Anorexia, nausea / vomiting, diarrhea. Category C in pregnancy
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Drug interactions with Metronidazole.
Displacement of bilirubin from albumin binding sites can increase risk of kernicterus (brain damage) in neonates and premature infants. Displacement of oral anticoagulants or sulfonylurea hypoglycemic agents from plasma proteins can increase the effect of the displaced drugs
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Intrinsic resistance
Intrinsic resistance occurs “just because” of the natural properties of the bacteria in question. An example would be vancomycin resistance of Gram-negative organisms due to the inability of vancomycin to penetrate the outer membrane of Gram-negative organisms. Other examples include E. coli resistance to penicillin but sensitive to ampicillin because their porins don’t allow penicillin into the cell, Mycoplasma resistance to beta-lactams because there is no cell wall, and Pseudomonas resistance to multiple agents.
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Acquired resistance
Acquired resistance develops by genetic mutation or by acquisition of new genes. New genetic material mediating antibiotic resistance is usually spread from cell to cell by way of mobile genetic elements such as plasmids, transposons, or bacteriophages.
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Antibiotic tolerance.
Organisms that resist killing or inhibition by antimicrobials to which they are inherently susceptible (Drug indifference). Such things include biofilms, metabolic bypass, anaerobic growth, and stationary phase.
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Major mechanisms of resistance:
In general, antibiotic resistance occurs through mechanisms that: Inactivate or modify the drug. Alter the antibacterial target. Reduce the ability of the drug to get to the target. Additionally, specific growth states (e.g. growth in a microbial biofilm, growth in anaerobic conditions, and stationery phase of growth) can also negatively impact antimicrobial susceptibility. These last examples can be important factors in reducing antibiotic success, and may help to explain why many courses of antibiotics have to be given for so long.
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Source of antibiotic resistance.
Antibiotic resistance genes were present far before antibiotics were ever used to combat infectious diseases. Since antimicrobial agents frequently come from microbes, antibiotic resistance probably originated from antibiotic-producing bacteria in the soil. Nowadays, antibiotic resistance is fueled in large part from the excessive use of antibiotics in feedlots and animal husbandry, and overuse in the medical community. Significant environmental levels of multiple classes of antibiotic agents are now common, which enhances development of antibiotic resistant genes that can disseminate to organisms more prone to cause disease in humans.
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Porins.
Found in the outer membrane of gram-negative bacteria (there is NO outer membrane in gram-positive bacteria). Porins form hydrophilic channels to allow the selective uptake of essential nutrients and other compounds, including some hydrophilic antibiotics. Changes in configuration or number of porin channels may adversely affect uptake of antibiotics.
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Efflux pumps.
Structures located in bacterial cell membranes that function to eliminate certain substrates from the bacterial cytoplasm. They originated as mechanisms to get rid of toxic substances from inside the bacterium, including antibiotics. (Similar in structure/function as P- glycoproteins in humans). Can be present in both gram-positive and gram-negative organisms. Can adversely affect uptake of antibiotics. Efflux pumps can be specific for a certain drug or class of drugs, or may be more general and result in multi-drug class resistance.
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Enterobacteriaceae (Enteric bacteria).
a large family of Gram-negative bacteria that includes, along with many harmless symbionts. They are responsible for many infections in the abdomen. Enterobacter, Escherichia (E. coli), Klebsiella, and (others). Commensal bacteria in the human bowel. Frequently associated with antibiotic resistance. Some of these pathogens have a propensity to hang around hospitals colonizing/infecting hospitalized patients.
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Non-Enterobacteriaceae (non-Enterics).
Gram negative rods. Pseudomonas, Stenotrophomonas , Acinetobacter, and (others). Usually infections acquired in the hospital and can be highly antibiotic resistant.
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Staph aureus with plasmids
Staph aureus has a plasmid that makes it resistant to penicillin and ampicillin. E coli has a plasmid that makes it resistant to ampicillin. Klebs pneum has a chromosome gene to ampicillin. Consequence: New abx were introduced (e.g., 3rd gen cephs). Point mutations in these narrow spectrum B-lactamases then conferred resistance to an “extended spectrum” of B-lactams.
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backbone of two alternating sugars, N-
Peptidoglycan Structure:acetylglucosamine (GlcNAc) and N-acetylmuramic acid (MurNAc) cross-linked via peptide bridge.
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Peptidoglycan Formation:
Peptidoglycan is formed by the addition of subunits called precursors (a GlcNAc- MurNAc disaccharide with its five attached amino acids on the MurNAc) that are assembled in the cytoplasm and transported through the cytoplasmic membrane to the cell surface.
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Peptidoglycan Cross-linking (transpeptidation):
is driven by cleavage of the terminal stem-peptide amino acid. The enzymes that perform cross-linking of peptidoglycan are called Penicillin Binding Proteins (PBPs)
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How do β-lactam antibiotics work?
Biosynthesis of peptidoglycan is accomplished by membrane-bound enzymes known as penicillin- binding proteins (PBPs). PBPs perform transpeptidase and/or transglycosylase reactions. These PBPs are involved in peptidoglycan synthesis during cell growth, cell septation and, in some species, sporulation. β-lactam antibiotics act by irreversibly binding to and inactivating the transpeptidase reaction of penicillin-binding proteins, thereby inhibiting peptide cross-linking and peptidoglycan synthesis.
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How do bacteria become resistant to β-lactams?
1) Modifying the drug—through enzymatic destruction of β-lactams by β-lactamases.
2) Modifying the target—through production of altered penicillin-binding proteins (PBPs) with low affinity for β-lactam binding.
3) Preventing drug-target interaction—both porin channel mutations and drug efflux mechanisms can increase resistance to β-lactams by preventing β-lactam interaction with the PBPs.
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β-lactamases.
β-lactamases are enzymes that inactivate β-lactam antibiotics by splitting the amide bond of the β-lactam ring, and thereby protect the activity of penicillin binding proteins.
β-lactamases are encoded either by chromosomal genes or by transferable genes located on plasmids or transposons. Can be found in BOTH gram-positive and gram-negative organisms. In general, the big bad. β-lactamases are found in gram-negative bacteria. There are hundreds of known β-lactamases, and they can be classified according to their structure or by their substrate profile (what types of β-lactams they can cleave). Additionally, some β-lactamase inhibitors (i.e sulbactam, tazobactam, clavulanate) will avidly bind and inactivate some β-lactamases, but they do NOT inhibit all β-lactamases (e.g. ampC-encoded β-lactamase) or carbapenemases.
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Narrow-spectrum β-lactamases.
These beta-lactamases generally hydrolyze penicillin-type antibiotics (penicillin, ampicillin, amoxicillin) but don’t have much activity against cephalosporins. Such bugs include staph, E coli, h. influenza, and Klebsiella pneumoniae. They can be found in both gram- positive and gram-negative organisms AND they can occur frequently (e.g. more than 95% of S. aureus isolates contain a narrow spectrum beta-lactamase) or less commonly like TEM-1 in E. coli. For gram-negative bacteria, these initial “emerging” resistant strains that elaborated narrow- spectrum β-lactamases, such as the TEM-1 of E. coli and SHV-1 of Klebsiella pneumoniae prompted the development of newer β-lactams (the extended spectrum β-lactams like 2nd and especially the 3rd generation cephalosporins).
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ESBLs (extended spectrum beta-lactamases).
Most “extended-spectrum β-lactamases” (ESBLs) began to arise in the 1980s and are mutants of TEM-1, TEM-2 and SHV-1 (“narrow” spectrum β-lactamases) with 1-to-4 amino acid sequence substitutions. The most notable feature of these enzymes, distinguishing them from their parent types, is their ability to attack most cephalosporins (like the 3rd generation cephalosporins—ceftriaxone and ceftazidime). ESBLs are usually found on plasmids, which means they are highly mobile and can disseminate, and are often present with additional genes encoding resistance to other classes of antibiotics. Found almost exclusively in Gram-negative rods, but prevalence is still pretty low (most isolates do NOT have them). Examples of these bugs include E coli and Klebsiella pheumonia.
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ampC-encoded β-lactamase.
ampC was first characterized as a chromosomally located gene in a number of gram-negative organisms. It encodes for a β-lactamse (technically a cephalosporinase) that is capable of hydrolyzing penicillins, and all 1st, 2nd, and 3rd-generation cephalosporins, and its activity is not inhibited by beta- lactamase inhibitors (ESBLs can be inhibited by some β-lactamase inhibitors but some question whether they should be used clinically). Found in the chromosome of certain Gram-negative rods: Enterobacter, Pseudomonas, and a few others.
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Is the chromosomal ampC beta-lactamase always expressed?
Processing peptidoglycan is an inducer that binds to repressor on ampC gene. Beta- lactams therefore can trigger these peptidoglycans to be produced. These peptidoglycans are made normaly but cleaned up from another enzyme. Mutation in this enzyme can cause it to be continually expressed. ampC is that its expression is either inducible (can be turned on) or constitutive (is on all the time). Under normal conditions, only trace amounts of AmpC are expressed, but this expression can be induced with some (not all) β-lactams, conferring inducible resistance to β-lactams like ampicillin and cefazolin (1st generation cephalosporin). Acquisition of some mutations can lead to constitutive expression of AmpC. When that occurs, ampC is expressed all the time and mediates resistance to all β-lactams except carbapenems. Enterobacter is one of the organisms that has ampC in its genome. Normally with ampC bugs are still suspetable to 3rd generation cephalosporin and tazobactam but with enterobacter it is resistant. The bug is resistant to ampicillin and cefazolin because both of those drugs induce ampC expression and are hydrolyzed by the beta-lactamase. Ceftriaxone, ceftazidime, and pipercillin/tazobactam do NOT induce ampC, so no β-lactamase is made, and the bug tests sensitive to these drugs. Although ertapenem induces high levels of ampC, it is not degraded by AmpC, so the Enterobacter is still sensitive to ertapenem. The third generation cephalosporins are now degraded by AmpC since it’s being expressed all the time. Pipercillin/tazobactam is also degraded because the ampC-encoded β-lactamase is NOT inhibited by the tazobactam. Even though ampC is on all the time now, the bug is still sensitive to ertapenem because the ampC enzyme does NOT degrade carbapenems well.
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What is the clinical significance of ampC?
The mutational events that lead to permanent high-level expression of ampC can occur during therapy! Thus, if you treated the first Enterobacter strain with ceftriaxone, you might select for a constitutive mutant during therapy. In general, if you want to be safe when treating organisms that house inducible ampC, you need to remember which organisms have it and then use a carbapenem.
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Carbapenemases.
Carbapenems (ertapenem, imipenem, meropenem, doripenem) are β-lactam antibiotics with the broadest spectrum of activity against most gram-negative rods and are used frequently in hospitalized patients when one worries about infections with potentially resistant gram-negative organisms. Gram- negative rods that are resistant to most β-lactams by virtue of ESBLs or ampC will generally be sensitive to carbapenems. Carbapenemases that are causing the most problem (KPC and NDM-1 as examples) are plasmid- mediated and are found mainly in some Klebsiella pneumoniae isolates, although transfer to E. coli and other Gram-negative rods have occurred.
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KPC :
Klebsiella pneumoniae carbapenemase “KPC” is an enzyme that hydrolyzes all carbapenems and all other β-lactams. First described in 2001 in North Carolina, KPC-mediated resistance is most commonly observed in the United States but it has spread to other parts of the world. Not common in Colorado yet, but 1/3 of all K. pneumoniae isolates in Brooklyn, NY medical centers are KPC producers. Found on plasmids.
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NDM-1 :
New Dehli metalo-betalactamase-1, NDM-1, hydrolyzes all β-lactams except aztreonam,. A number of reports have documented the widespread occurrence of NDM-1 in both India and Pakistan. Our University Hospital experienced an outbreak with an NDM-1 producing Klebsiella pneumoniae strain in 2014. Found on plasmids.
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Clinical correlate of Carbapenemases:
Although carbapenemase-containing gram-negative rods are a huge worry, some gram-negative organisms can become resistant to carbapenems without a carbapenemase. Through a combination of porin channel mutations, efflux activation and/or overexpression of some beta- lactamases, resistance to carbapenems may develop. You will see this most commonly with some Pseudomonas isolates that are carbapenem resistant. Most likely, this will NOT be due to a carbapenemase.
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Altered penicillin-binding proteins (PBPs).
A second mechanism of bacterial resistance to β-lactam antibiotics is production of a PBP that has a markedly reduced affinity for the drug (beta-lactams). While this alteration may occur by mutation of existing genes, the acquisition of new PBP genes or new pieces of PBP genes is more important. Such bugs include Staphylococcis spp, S. pneumonia, and N. gonorrhoeae.
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mecA :
Only seen in staphylococci (about 30-40% of staphylococci). Gene encodes for a low affinity penicillin-binding protein (PBP) called PBP2a. This modified penicillin-binding protein confers resistance to all beta-lactam agents (except the brand new 5th generation cephalosporins). When present in S. aureus, it is classified as “methicillin-resistant” S. aureus (40-50%), otherwise known as MRSA.
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Mosaic PBPs :
Seen mainly in Streptococcus pneumoniae and Neisseria gonorrhoeae (R not because beta-lactamase). These bacteria can “pick up” pieces of genetic material (via transformation) from other bacteria and swap them with similar pieces of DNA in their own chromosome. The end result is that genes encoding PBPs in these organisms can become “mosaics” over time, which may encode for PBPs with markedly reduced affinity for beta- lactam antibiotics.
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How does vancomycin work?
Vancomycin targets the peptidoglycan precursor molecule by binding to the terminal D-alanine-D-alanine portion of the five-member peptide chain. By doing so, vancomycin prohibits incorporation of the precursor molecule into the pedtidoglycan. This disrupts cell wall synthesis and leads to cell death.
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How do bacteria become resistant to vancomycin?
In terms of broad categories, bacteria can become resistant to vancomycin by:
1) Modifying the target—seen almost exclusively in Enterococcus spp, through acquisition of a plasmid that carries a gene that will change the terminal five-member peptide that hangs off of the MurNAc sugar, making the precursor unrecognizable by vancomycin.
2) Preventing drug-target interaction—rather complicated and interesting mechanism seen mainly in S. aureus that prevents vancomycin interaction with the peptidoglycan precursor by binding free vancomycin in the existing peptidoglycan cell wall.
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Enterococcus and high-level resistance to vancomycin.
Vancomycin-resistant enterococci (VRE) were first described in 1988 and have disseminated widely. Genes encoding resistance (commonly vanA or vanB) are carried on plasmids. Enzymes encoded by vanA or vanB substitute D-lactate for the terminal D-alanine on the 5- member peptide chain of the peptidoglycan precursor, resulting in synthesis of precursor peptidoglycan molecules with peptide chains terminating as D-alanine-D-lactate. The modified peptidoglycan precursor binds vancomycin with reduced affinity, conferring high-level clinical resistance to vancomycin. The plasmid can transfer to other Enterococcus spp. by conjugation.
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Clinical correlate with VRE:
The great concern about ten years ago or so, was that since vancomycin resistance in enterococci was encoded on a plasmid, that plasmid might get into S. aureus, producing a vancomycin-resistant S. aureus strain. Although this has occurred about 11 times, there has NOT been widespread dissemination of these vancomycin-resistant S. aureus (VRSA) isolates.
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Staphylococcus and reduced susceptibility to vancomycin.
A few isolates of S. aureus (and even more rare coagulase-negative staphylococci) have been found that display a moderate reduction in vancomycin susceptibility (not fully “resistant”). Such isolates are called vancomycin-intermediate-susceptibility S. aureus strains or VISA. Many more isolates may contain subpopulations with reduced vancomycin susceptibility (heteroVISA or hVISA). These isolates have NOT acquired the genes that mediate vancomycin resistance. Rather, they have perturbations in their cell wall synthesis leading to two major differences compared with sensitive strains: Cell walls contain markedly thickened layers of peptidoglycan. There is LESS cross-linking of the peptides. This leaves free amino acid chains in the cell wall that still contain the D-alanine – D-alanine terminus, where vancomycin can bind. The end result is that vancomycin gets bound up in the cell wall (which does NOT cause cell death) and leaves less free vancomycin to bind to the precursor peptidoglycan molecules, which is necessary for vancomycin’s antibacterial effect.
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VISA or hVISA
VISA or hVISA isolates usually have developed in patients while on prolonged vancomycin therapy. Usually this occurs with long-standing and/or repetitive treatment with vancomycin in a single patient. The continued vancomycin pressure on S. aureus leads to complex changes in how S. aureus synthesizes its cell wall. Understand that making a thick cell wall with decreased cross-links is a complex process that probably involves differential expression of a number of genes. Because of the complex nature of resistance in VISA strains, we don’t see VISA strains very often despite our unbelievably frequent usage of vancomycin in the hospital.
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How do quinolones work?
Quinolones (ciprofloxacin, levofloxacin, moxifloxacin) target the essential bacterial enzymes DNA gyrase and DNA topoisomerase IV. Both are large, complex enzymes composed of 2 pairs of subunits. The subunits of DNA gyrase are GyrA and GyrB, while the subunits of topoisomerase IV are ParC and ParE. The two enzymes function during replication, transcription, recombination and repair of DNA. Quinolones trap DNA gyrase or topoisomerase IV as a drug-enzyme-DNA complex, with subsequent release of lethal, double–stranded DNA breaks.
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Clinical correlate with quinolone:
Each quinolone has slightly different preferential activity against either DNA gyrase or topoisomerase IV, which helps to explain their respective antimicrobial activities. This is due to the fact that in gram-negative bacteria, DNA gyrase is more susceptible to inhibition by quinolones than is topoisomerase IV, whereas, in gram-positive bacteria, topoisomerase IV is usually the prime target.
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Ciprofloxacin
Ciprofloxacin preferentially targets DNA gyrase over topoisomerase IV, ciprofloxacin is much more active against gram-negative bacteria than gram-positive organisms.
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Levofloxacin
Levofloxacin targets DNA gyrase about as well as ciprofloxacin, but has much better activity against topoisomerase IV, levofloxacin has good gram-negative activity and significantly improved gram-positive activity.
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Moxifloxacin
Moxifloxacin targets topoisomerase IV preferentially; moxifloxacin has probably the best activity against gram-positive organisms while still retaining some gram-negative activity (although NOT against Pseudomonas aeruginosa).
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How do bacteria become resistant to fluoroquinolones?
In terms of broad categories, bacteria can become resistant to fluoroquinolones by:
1) Modifying the drug—very rare cause of resistance although newly emerging.
2) Modifying the target—through key amino acid changes (caused by DNA point mutations) in the subunits of DNA gyrase and/or topoisomerase IV.
2) Preventing drug-target interaction—drug efflux mechanisms (and to a lesser degree porin channel mutations in gram-negative bacteria) can increase resistance to fluoroquinolones by preventing their interaction with DNA gyrase or topoisomerase IV.
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Modifying the target with quinolone resistance.
Quinolone resistance occurs due to one or more nucleotide mutations that result in amino acid substitutions in a region of GyrA of DNA gyrase, or ParC of topoisomerase IV called the quinolone-resistance-determining region (QRDR). These amino acid changes make the enzyme less sensitive to inhibition by quinolones, most commonly by reducing the affinity of the enzyme-DNA complex for the quinolone (i.e. quinolones have trouble binding to the enzyme).
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How do macrolides work?
Macrolides are a class of antibiotics including erythromycin, clarithromycin and azithromycin (you will use azithromycin most often). Macrolides inhibit bacterial protein synthesis through binding to a domain of the 23S ribosomal RNA that is a component of the 50S subunit of the bacterial ribosome. Macrolide binding to the ribosome prevents peptide chain elongation.
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Where will we use macrolides the most?
Macrolides are good drugs to use to treat patients with mild community-acquired pneumonia, because they will cover most all of the respiratory bacteria that might cause infection (including Streptococcus pneumoniae, a very common etiology). As well, macrolides can be used to treat inner ear infections in kids (acute otitis media—S. pneumoniae is common here too) and for Strep throat (caused by Streptococcus pyogenes—group A strep) in patients allergic to penicillin.
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How do bacterial species become resistant to macrolides?
1) Modifying the drug—very rare cause of resistance in some gram-negative rods and S. aureus. (Not important to the discussion below.)
2) Modifying the target—very common cause of macrolide resistance most commonly through expression of an enzyme that dimethylates (puts 2 methyl groups on) 23S ribosomal RNA and prevents macrolide binding to the bacterial ribosome. Mediated by erm (erythromycin ribosome methylase) gene that encodes the dimethylase enzyme, which is usually found on plasmids or transposons.
3) Preventing drug-target interaction—another common cause of macrolide resistance through chromosomal or plasmid encoded efflux pumps.
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Modifying the target with marcolids resistance.
Ribosomal dimethylation is performed by a methylase encoded by erm.
Methylation of the ribosome confers resistance to macrolides AND another antibiotic called clindamycin.
erm regulation can be inducible or constitutive. It is induced by macrolides ONLY, not by clindamycin. For isolates with constitutive expression of erm (on all the time): they will test resistant to macrolides and clindamycin. For isolates with inducible expression of erm (induced in the presence of macrolides): they will test resistant to macrolides and sensitive to clindamycin. However, mutations in the inducible system can easily occur, leading to constitutive expression of erm. When this happens, the bacteria will lose sensitivity to clindamycin.
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Efflux with microlides resistance
. Mediates resistance to macrolides. Isolates with an efflux pump will test resistant to macrolides and sensitive to clindamycin.
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How do bacterial species become resistant to aminoglycosides?
1) Modifying the drug—classic mechanisms and probably board-test worthy information to know. Enzymes capable of covalently modifying aminoglycosides are found on transmissible plasmids, frequently within transposons, or in the chromosome. Observed in both Gram- negative and Gram-positive organisms.
2) Modifying the target—common cause of aminoglycoside resistance most commonly through expression of an enzyme that methylates a site of the 16S rRNA at which aminoglycosides bind. Carried on plasmids. 3) Preventing drug-target interaction—uptake of aminoglycosides is dependent upon a sufficient electrochemical gradient established by the activity of the bacterial electron transport chain. Therefore, anaerobic bacteria, which do not have an aerobic respiratory transport chain system, do not have the electrochemical gradient necessary to get the aminoglycosides into their cells. Thus, in general, anaerobic bacteria are resistant to aminoglycosides.
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Modifying the drug with resistance to aminoglycoside:
aminoglycoside modifying enzymes.
More than 30 aminoglycoside-modifying enzymes that have been identified are capable of three general reactions: N-acetylation, O-nucleotidylation, and O-phosphorylation. Modification of aminoglycosides by these enzymes inactivates the drug and leads to high level resistance.
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What is the need for a D-test?
Isolates harboring either efflux-mediated resistance or inducible ribosomal methylation will test the same: resistant to macrolides and sensitive to clindamycin. For streptococci and staphylococci the following phenotype can be explained through 2 different mechanisms: Erythromycin resistance and Clindamycin sensitive.
Either the bacterium has an inducible erm methylase system or a macrolide efflux pump system. In the first case, one would be wary of using clindamycin, because if a mutation occurs in the bacterial population while treating with clindamycin, that would cause constitutive expression of erm and resistance to clindamycin. In the second scenario with the efflux pump, clindamycin use should be ok, as efflux will only act on the macrolide. So, how can you tell the difference between the systems?
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The D-test:
An agar plate is streaked with the bacteria that tests Erythromycin R/Clindamycin S. Then, a disk containing erythromycin is placed near a disk containing clindamycin. If the bacteria in question possesses the inducible erm system, erythromycin will induce resistance to clindamycin and the zone of clearing for clindamycin will be blunted, forming a D-shape of colony growth. If the efflux system is present, no such blunting will occur. Bottom line is that for organisms harboring erm that test sensitive to clindamycin, you shouldn’t use EITHER macrolides or clindamycin. If the organism harbors an efflux pump to macrolides, you shouldn’t use macrolides, but clindamycin is fine.
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Empiric therapy with antibiotics:
In most cases, it is impossible to determine the exact nature of the infecting organism(s) before the institution of antimicrobial therapy. In these cases, one must apply knowledge of the organisms most likely to cause infection in a given clinical setting to use empiric antibiotics appropriately.
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Empiric treatment with life-threatening infection.
Therapy is usually begun with more than one agent and is later tailored to a specific pathogen or pathogens if eventually identified. Earlier initiation of antibiotic therapy has correlated with improved survival for many infectious syndromes (eg meningitis).
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Empiric treatment with outpatient treatment of infections.
In many situations, you will be treating infectious syndromes before culture results (eg some urinary tract infections) or even without obtaining cultures at all (eg otitis media).
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Antibiogram:
Awareness of local susceptibility patterns is useful when the patient is treated empirically. Antibiotics are frequently used empirically, prior to having any susceptibility information. An important tool in helping to choose empirical antibiotic therapy is a local antibiogram report. Denver Health, the University of Colorado Hospital and the Denver VA publish a yearly antibiogram. This report is a yearly summary of local susceptibility patterns for common pathogens encountered throughout the year. The tables list specific pathogens and the percent of strains susceptible to different antimicrobials to help in choosing appropriate antibiotics.
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Disk diffusion (Kirby-Bauer)
Disk diffusion (Kirby-Bauer) in testing bacterial susceptibility— A suspension of the isolate at a specified concentration is prepared, and then spread evenly onto a large agar plate. Disks impregnated with defined concentrations of an antibiotic are placed onto the surface of the agar. After incubation, the diameter of the clearing around the disk (area of no bacterial growth) is measured.
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E test with testing bacterial susceptability—
A variation of the Kirby-Bauer approach is to use a strip impregnated along its length with a concentration gradient of an antibiotic—the strip is placed onto an agar plate that has been spread with a suspension of the isolate of interest. After incubation and growth of the organism, an ellipse shaped area of no clearing around the strip occurs. Where the ellipse meets the strip, an MIC can be determined. The strips are easy to read and the MIC value can be determined directly from the strip.
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Broth dilution in testing bacterial susceptibility—
Serial dilutions of an antibiotic are made in a liquid medium, which is inoculated with a standardized number of organisms and incubated for a prescribed time. The tubes/wells are visually examined (or are visualized in an automated system) to see if there is evidence of bacterial growth. After incubation the tube/well with the lowest concentration of antibiotic that prevents visible bacterial growth is determined. This value is called the Minimum Inhibitory Concentration or MIC.
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Minimal bactericidal concentration (MBC):
This is another important term and this is derived from an extension of MIC testing. The MBC is the lowest concentration of the antibiotic that kills 99.9% of the original inoculum in a given time. Is used to determine whether a certain drug is considered bactericidal (actively kills) or bacteriostatic (inhibits growth) against a certain bacteria.
Bactericidal antibiotics have MBC concentrations that are equal to or just above the MIC.
Bacteriostatic antibiotics have MBC concentrations many fold higher than the MIC concentration. NOT used in clinical microbiology labs today. Is NOT used in determining drug susceptibility.
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Testing for antimicrobial resistance:
```
besides performing susceptibility tests, many microbiology labs will specifically test and/or screen for the presence of certain antibiotic resistant mechanisms.
Some bacteria (ex H. influenzae) will be tested for beta-lactamase activity.
```
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Susceptibility interpretation:
The laboratory provides susceptibility reports by categorizing isolates as susceptible, intermediate (or sometime susceptible-dose dependent), or resistant based on the determined MIC (via broth dilution or Etest) or diameter of clearing around the disk (Kirby-Bauer) for an antibiotic tested against a bacteria. The definitions of these categories are susceptible, intermediate, susceptible –dose dependent, and resistant.
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Susceptible (S) bacteria:
susceptible implies that infection due to the bacteria tested will probably respond to that antibiotic.
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Intermediate (I) bacteria:
intermediate implies an indeterminate or uncertain response is likely given standard dosing. In some circumstances, increased does might be effective.
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Susceptible-dose dependent (SDD) bacteria:
SDD implies that susceptibility of an isolate is dependent on the dosing regimen that is used in the patient. In order to achieve levels that are likely to be clinically effective against isolates in the SDD category, it is necessary to use a dosing regimen that results in higher drug exposure than the dose that was used to establish the susceptible breakpoint. Application of SDD usually requires multiple approved dosing options for a given antibiotic.
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Resistant (R) bacteria:
resistant implies that infection due to bacteria tested will probably not respond to that antibiotic.
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Interpretation caveats:
Remember that these breakpoints are generally: NOT adjusted for site of infections (i.e. ability of antibiotics to penetrate into the site of
infection). NOT adjusted for the number of organisms in an infection (i.e. a closed space abscess,
which is unlikely to respond to any amount of antibiotics unless it is drained first). NOT adjusted to the conditions in the host (i.e. the pH at the site of infection might be low and not amenable to aminoglycoside therapy, as aminoglycosides don’t work in
low pH.). NOT adjusted to reflect the patient’s host defenses.
Thus—any one of the above scenarios are variables that can affect the clinical outcome of patients treated for infection that are INDEPENDENT of whether a bacteria is considered susceptible or resistant to a given drug
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Setting breakpoints:
Established by Clinical and Laboratory Standards Institute (CLSI). Breakpoints are established such that for a given MIC or zone diameter of inhibition, a category (S, I, R) can be assigned. Generally, these are some of the issues considered in setting breakpoints.
The pattern of susceptibility of a population of bacteria. The pharmacokinetics of the antimicrobial. Clinical experience—many times evidence will surface revealing clinical failures of a certain antibiotic against a certain bacteria that has tested “susceptible” to that antibiotic. The established breakpoints may then be lowered. In GENERAL, for a bacterium to be considered susceptible to an antibiotic:
At normal dosages, the antibiotic should have a maximal serum concentration that exceeds the MIC of the bacteria (Cp > MIC)
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Choosing antibacterial agents.
Whether using antibiotics empirically or with knowledge of the infecting organism(s) and their susceptibilities, the choice of antibacterial agent(s) is guided by the pharmacokinetic and adverse-reaction profile of active compounds, the site of infection, the immune status of the host, and evidence of efficacy from well-performed clinical trials. Once the organisms(s) is identified and its susceptibility to antibacterial agents is determined, the regimen with the narrowest effective spectrum should be chosen. It is estimated that ~ 50% of antibiotic use is in some way inappropriate. “Right drug at the right time at the right dose for the right duration”
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Local factors of an infected site.
The location of the infected site frequently plays a major role in the choice and dose and route of administration of antimicrobial drugs. Remember, it is often difficult to correlate therapeutic outcome with in vitro susceptibility
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Distribution
of antibiotics.
For antibiotics to work, local concentrations should be at least = MIC. Most infections are extravascular and so antibiotics have to be delivered to the site of
infection. Although plasma concentrations may seem adequate, tissue levels may be
poor (ex Vancomycin in treatment of S. aureus pneumonia). iv vs oral: Many infections require rapid onset of effective antibiotic therapy (ie. time
to peak plasma concentrations-French). This is usually achieved much more readily and with higher peak concentrations if iv infusions are used instead of the oral route.
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Protected local environments.
Includes meningitis, bone infections, endocarditis, intracellular niche, abscesses, and foreign bodies.
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Meningitis:
Recall the tight junctions in the blood-brain barrier (French). These have to be crossed in order to achieve an adequate concentration of antibiotic at the site of infection. Also recall how the endothelial cell barrier becomes more “leaky” in the presence of inflammation, which allows some drugs to reach higher concentrations in the cerebral spinal fluid than observed without inflammation. In addition, bacteria in the meninges are protected from host defense mechanisms and so bactericidal therapy is recommended.
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Bone infections:
bacteria in bone are relatively protected from opsonophagocytic removal. Prolonged antibiotic therapy is usually required. Some antibiotics (eg fluoroquinolones) concentrate in bone and use may result in better clinical outcomes.
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Endocarditis
(heart valve infection): The mixture of platelets and fibrin create a protected niche for bacteria away from host defense mechanisms and so bactericidal therapy is recommended.
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Intracellular niche:
Many bacterial pathogens can survive and even thrive inside of cells (including phagocytes!!). Thus, antibiotics will need to penetrate the intracellular environment to treat pathogens capable of intracellular existence. Some are better at this (eg rifampin, tetracyclines, erythromycin) than others (eg aminoglycosides, beta-lactam agents).
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Abscesses:
Penetration of antibiotics into large collections of fibropurulent inflammation and bacteria may be difficult. Some antibiotics are bound up and inactivated in this milieu (eg aminoglycosides). As well, local O2 concentrations may be low in these environments, which may adversely affect the sensitivity of organisms to some antibiotics. Thus, one of the guiding principles of effective antibiotic therapy is that abscesses are a surgical disease: surgical drainage is generally necessary with large abscesses for good clinical outcome!!!!
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Foreign bodies
(eg central catheters, Foley catheters, pacemakers, artificial heart valves, artificial joints, surgical mesh etc). Treatment of infections with associated foreign bodies is generally unsuccessful unless the foreign body is removed. Foreign bodies impair local host defenses (recall frustrated phagocytosis—Knez) • Foreign bodies are a nidus for organisms’ adherence/biofilm formation, which are poorly treated with antibiotics alone.
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Bladder with antibiotics.
Some drugs concentrate readily in the bladder, achieving concentrations 100x higher
than in blood. Since blood is the usual reference fluid in defining susceptibility, even organisms found
to be resistant to achievable serum concentrations may be susceptible to achievable
urine concentrations. There are some drugs that penetrate poorly into the urine (i.e. moxifloxacin), so that
although quinolones as a group are excellent at treating urinary tract infections, moxifloxacin would not be a good choice in treating cystitis (bladder infection). Most often E coli.
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Daptomycin and pulmonary surfactant:
Daptomycin was shown to penetrate well into epithelial lining fluid of the lung.
Daptomycin is highly active against S. pneumoniae in vitro. Daptomycin was found to be less effective than standard treatment in clinical trials of
pneumonia. It was later discovered that daptomycin is inactivated by pulmonary surfactant and is
not recommended for treatment of pneumonia
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Host Factors with choosing antibiotics.
1) History of previous adverse reactions to antibiotics. 2) Renal function/liver function. 3) Age-neonatal liver does not handle bilirubin well. Sulfonamides
compete with bilirubin for binding to albumin (sulfonamides will displace bilirubin,
leading to higher serum concnetrations) and can lead to kernicterus. Tetracyclines avidly bind to developing teeth and bones and can cause damage or
discoloration in young children.
4) Genetic/ metabolic factors- Patient’s with G6PD deficiency can develop hemolysis when treated with certain
antibiotics (eg dapsone, sulfonamides, some anti-malarial compounds). Fluoroquinolones can lead to hypo/hyperglycemia that may put patients with diabetes
mellitus at greater risk. 5) Pregnancy.
6) Drug interactions: need to look through patient’s drug lists to evaluate. Luckily, pharmacists are available and pro-actively do this as part of their job. Electronic ordering will also alert you to drug-drug interactions.
7) Immune status: For patient’s undergoing certain types of chemotherapy, profoundly decreased levels of
circulating neutrophils may be observed. In these cases, clearance of a bacterial
infection is dependent on the antibiotics alone, so bactericidal therapy is recommended. Additionally, patient’s on immunosuppressive medications may have infecting organisms not usually seen in otherwise healthy individuals, such that antimicrobial agents need to
be chosen carefully (eg patient with fungal pneumonia)
