ch 14 - antimicrobial drugs Flashcards
use of antimicrobials in ancient societies
there is evidence that humans have been exposed to antimicrobial compounds for millennia, not just in the last century
- antimicrobial properties of certain plants were also recognized by various cultures
ancient brewers tapped antibiotic secrets
ancient Egyptians and Jordanians used beer to treat gum disease and other ailments
a chemical analysis of the bones of ancient Nubians shows that they were regularly consuming tetracycline
- most likely in their beer
- this is the strongest evidence that the art of making antibiotics which dates to the discovery of penicillin in 1928, was common practice nearly 2,000 years ago
first antimicrobial drugs timeline
the first half of the 20th century was an era of strategic drug discovery
early 1900s
- Paul Ehrlich and his assistant Sahachiro Hata found compound 606
- killed Treponema pallidum
- sold under the name of Salvarsan
1928
- Alexander Fleming discovered penicillin, the first natural antibiotic
1930s
- Klarer, Mietzsch, and Domagk disovered prontosil
- killed streptococcal and staphylococcal infections
- the active breakdown product of prontosil is sulfanilamide
- Sulfanilamide was the first synthetic antimicrobial created
early 1940s
- Dorothy Hodgkin determined the structure of penicillin using x-rays
- scientists could then modify it to produce semisynthetic penicillins
1940s
- Selman Waksman’s research team discovered several antimicrobials produced by soil microorganisms
first antimicrobial drugs: early 1900s
Paul Ehrlich and his assistant Sahachiro Hata found compound 606
- sold under the name of Salvarsan
compound 606 (Salvarsan)
- kills Treponema pallidum
first antimicrobial drugs: 1928
Alexander Fleming discovered penicillin
- first natural antibiotic
first antimicrobial drugs: 1930s
Klarer, Mietzsch, and Domagk disovered prontosil
- prontosil kills streptococcal and staphylococcal infections
prontosil’s active breakdown product is sulfanilamide
- sulfanilamide was the first synthetic antimicrobial created
first antimicrobial drugs: early 1940s
Dorothy Hodgkin determined the structure of penicillin using x-rays
- scientists could then modify it to produce semisynthetic penicillins
first antimicrobial drugs: 1940
Selman Waksman’s research team discovered several antimicrobials produced by soil microorganisms
chemotherapeutic agent/drug
any chemical agent used in medical practice
antibiotic agent
usually considered to be a chemical substance made by a microorganism
- can inhibit the growth or kill microorganisms
antimicrobic/antimicrobial agent
a chemical substance similar to an antibiotic
- but may be synthetic
antibiotic
usually has one bacterial target
- e.g. a key bacterial enzyme is blocked
develops too rapidly to be sustainable
- targets more than just DNA/membranes
antimicrobial
a broad term but often can mean multiple targets
- e.g. membranes and DNA
selective toxicity
harms microbes but not damaging the host
- e.g. harming the cell wall
antibiotics exhibit selective toxicity
chemotherapeutic index
the ratio between toxic dose to therapeutic dose
- the higher the chemotherapeutic index, the safer the drug
spectrum of antimicrobial activity
no single chemotherapeutic agent affects all microbes
antimicrobial drugs classified based on the type of organism they affect
- e.g. antibacterial, antifungal, etc
even within a group,
- one agent may have a narrow spectrum of activity
- whereas another many affect many species
types of spectrums of antimicrobial activity
- narrow spectrum
- broad spectrum
narrow spectrum of antimicrobial activity
targets only specific subsets of bacterial pathogens
broad spectrum of antimicrobial activity
targets a wide variety of bacterial pathogens
- including Gram-positive and Gram-negative species
opportunistic pathogen
microbes that usually do not cause disease in healthy people
- may become virulent with immunocompromised and unhealthy individuals
development of superinfections
- normal microbiota keeps opportunistic pathogens in check
- broad-spectrum antibiotics kill nonresistant cells
- opportunistic pathogens still remain - drug-resistant pathogens proliferate → can cause superinfection
types of antibiotic activity
- bacteriostatic
- bactericidal
- bacteriolytic
bacteriostatic
having the ability to inhibit bacterial growth
- generally by means of chemical or physical treatment
- reversible inhibition of a microbe’s ability to divide
total cell count & viable cell count
- increases logarithmically until reaching a still level
- this limits growth
bacteriocidal
irreversible inhibition of a microbe’s ability to divide
- dead cells are not destroyed
- total cell numbers remain constant
graph:
- total cell count and viable cell count increase logarithmically
- total cell count then hits a standstill
- viable cell count decreases logarithmically at the same time, getting killed
bacteriolytic
destruction or disintegration of bacteria
- lysis decreases both viable cell number & total cell number
graph:
- total cell count and viable cell count increase logarithmically
- then both decrease logarithmically over time
in vitro effectiveness
the effectiveness of an agent in a test tube or artificial environment
- determined by how little of it is needed to stop growth
the in vitro effectiveness of an agent is determined by…
how little of it is needed to stop growth
how is in vitro effectiveness measured
in terms of the antibiotic’s minimal inhibitory concentration (MIC)
minimal inhibitory concentration (MIC)
the lowest concentration of the drug that will prevent the growth of an organism
minimal inhibitory concentration (MIC) reflects…
antibiotic efficacy
minimal bactericidal concentration (MBC)
lowest concentration of antibacterial agent required to kill bacterium over fixed time period
- determined by doing serial tube dilution for antibiotic
can distinguish if a drug is bactericidal or bacteriostatic
- e.g. if cells grow in fresh medium without antibiotic → drug is bacteriostatic
determining minimum bactericidal concentration (MBC)
must use a serial tube dilution test for the antibiotic
- tubes with no visible growth are then inoculated onto agar media to determine MBC
types of antibiotic activity tests
- Kirby-Bauer disk susceptibility test
- E-test
types of antibiotic activity: Kirby-Bauer disk susceptibility test
determines how susceptible a bacteria is to several antibiotics
- this is shown by the zone of inhibition
- cannot distinguish whether a drug is bacteriostatic or bactericidal
antibiotic diffuses from the circular disk into the agar
- interacts with the growing bacteria
zone of inhibition is measured around filter-paper disks impregnated with antibiotics
e.g. the oxacillin disk
- MRSA has no zone of clearing because MRSA is resistant
- versus MSSA which is not resistant
types of antibiotic activity: E-test
determines MIC
- the intersection of the elliptical zone of clearing with the gradient on the drug containing strip indicates MIC
- cannot determine whether a drug is bacteriostatic or bactericidal
numbers on strip reflect the relative concentrations of antibiotic present at various points within the zone of inhibition
- the concentrations along the periphery of the clear zone are equal and reflect the MIC
which test can distinguish whether the drug is bacteriostatic or bactericidal, the MIC test or the Kirby-Bauer test?
neither, need MBC to do this
must do serial tube dilution of antibiotic
- tubes with no visible growth are inoculated onto agar media without antibiotic to determine MBC
agar plate determines if any cells survived 3x-5x above the MIC
→ if cells grow: drug is bacteriostatic
→ if cells don’t grow: drug is bactericidal
how to distinguish between bacteriostatic and bactericidal drugs
bacteriostatic
- if cells grow in the fresh medium without antibiotic
bactericidal
- if cells do not grow in the fresh medium without antibiotic
what are the 8 attributes of an ideal antimicrobial
- solubility
- selective toxicity
- toxicity not easily altered
- non-allergenic
- stability
- resistance by microorganisms not easily acquired
- long shelf-life
- reasonable cost
dosage
amount of medication given during a certain time interval
- route of administration must be considered
- half-life of antibiotic must be considered
in children
- dosage is based upon the patient’s mass
in adults
- a standard dosage is used, independent of mass
half-life of an antibiotic
the rate at which 50% of a drug is eliminated from the plasma
half-life of a drug
the rate at which 50% of a drug stays in tissue
when deciding which antibiotic to prescribe, the clinician needs to keep these three things in mind
- whether the organism is susceptible to the antibiotic
- whether the attainable tissue level of the antibiotic is higher than the MIC
- depends on the body organs - the understanding of the relationship between the therapeutic dose and the toxic dose of the drug
therapeutic dose
the minimum dose per kilogram of body weight that stops pathogen growth
toxic dose
the maximum dose that the patient can tolerate
route of administration: plasma concentration of drug as a function of response time
routes of administration:
1. IV (intravenous)
- reach peak level of plasma concentration of a drug immediately after t0
- and the magnitude is greatest out of all 3
2. IM (intramuscular)
- reach peak level of plasma concentration of drug at t1
- magnitude around half of IV’s
3. oral
- reach peak level of plasma concentration of a drug shortly after t1
- magnitude slightly lower than IM
- this is in relation to the chemotherapeutic index
combinations of antibiotics can either be
- synergistic
- antagonistic
synergistic drugs may work poorly when…
they are given individually
- when combined, they work very well
combined effect is greater than additive effect
e.g. aminoglycoside + vancomycin
antagonistic drugs may work poorly when…
they are combined with other antagonistic drugs
- mechanisms of actions will interfere with each other → diminish their effectiveness
e.g. penicillin + macrolides
antibiotics exhibit selective toxicity because…
they disturb enzymes or structures unique to the target cell
mechanisms include:
1. cell wall synthesis
2. cell membrane integrity
3. DNA synthesis
4. RNA synthesis
5. protein synthesis
6. metabolism
modes of action of antibiotics: targets of antibiotics
- cell wall
- plasma membrane
- DNA synthesis
- RNA synthesis
- ribosomes
- metabolic pathways
antibiotics that target: cell wall
1. β-lactams
- penicillins
- cephalosporins
- monobactams
- carbapenems
2. glycopeptides
- vancomycin
3. bacitracin
antibiotics that target: DNA synthesis
1. fluoroquinolones
- ciprofloxacin
- levofloxacin
- moxifloxacin
antibiotics that target: RNA synthesis
1. rifamycins
- rifampin
antibiotics that target: plasma membrane
1. polymyxins
- polymyxin B
- colistin
2. lipopeptide
- daptomycin
antibiotics that target: ribosomes
30S subunit
- aminoglycosides
- tetracyclines
50S subunit
- macrolides
- lincosamides
- chloramphenicol
- oxazolidinones
targets of antibiotics: metabolic pathways
folic acid synthesis
- sulfonamides
- sulfones
- trimethoprim
mycolic acid synthesis
- izoniazid
antibiotics that target the cell wall: penicillins
the enzymes that attach the disaccharide units to preexisting peptidoglycan and produce peptide cross links
- collectively called penicillin-binding proteins (PBPs)
without an intact cell wall, the growing cell eventually bursts
- due to osmotic effects
penicillin is a bactericidal drug
- type of β-lactam antibiotic (has a β-lactam ring)
antibiotics that target the cell wall: cephalosporins
β-lactam antibiotic originally discovered in nature but modified in the laboratory
- a type of semisynthetic drug
the basic structure of cephalosporin has been modified
- these modifications improve the drug’s effectiveness against penicillin-resistant pathogens
each modification is a new “generation” of cephalosporins
- there are currently 5 generations of this antibiotic
generation 1: Cephalexin
generation 2: Cefoxitin
generation 3: Ceftriaxone
generation 4: Cefepime
generation 5: Ceftaroline
inhibitors of cell wall synthesis
polypeptide antibiotics
- bacitracin
- vancomycin
antimycobacterial antibiotics
- isoniazid (INH)
- ethambutol
polypeptide antibiotics: bacitracin
topical application, works against Gram-positives
- form of inhibitor of cell wall synthesis
polypeptide antibiotics: vancomycin
glycopeptide
- important “last line” against antibiotic-resistant S. aureus
- form of inhibitor of cell wall synthesis
antimycobacterial antibiotics: isoniazid (INH)
inhibits mycolic acid synthesis
- form of inhibitor of cell wall synthesis
antimycobacterial antibiotic: ethambutol
inhibits incorporation of mycolic acid
- form of inhibitor of cell wall synthesis
microbial resistance to cell wall: inhibiting antibiotics
β-Lactamase (enzyme) is found in some bacteria
- it can break a bond in the β-lactam ring of penicillin → disables molecule
bacteria with β-Lactamase can resist the effects of penicillin
- and some other β-lactam antibiotics
Penicillin (w/ β-lactam ring) → (β-Lactamase) → Penicilloic acid
antibiotics that target the bacterial membrane: polymyxin, tyrocidin, and platansimycin
act as detergents and disrupt the structure of the cell membrane
- done by binding to the phospholipids
- highly toxic
e.g. Platensimycin
mode of action
- interacts with lipopolysaccharide in the outer membrane of Gram-negative bacteria
- this kills the cell through the eventual disruption of the outer membrane and cytoplasmic membrane
antibiotics that target the bacteria membrane: gramicidin
type of cyclic peptide
mode of action
- inserts into the cytoplasmic membrane of Gram-positive bacteria
- disrupting the membrane and killing the cell
can bacterial DNA synthesis be a target for antibiotics?
yes, there are a few unique features of bacterial DNA synthesis that makes it a target for antibiotics
antibiotics that affect DNA synthesis and integrity: metronidazole
metronidazole is activated after being metabolized by microbial protein cofactors ferredoxin
- ferredoxin is found in anaerobic and microaerophilic bacteria such as Bacterioides and Fusobacterium
aerobic microbes are resistant
- this is because they do not possess the electron transport proteins capable of reducing metronidazole
prodrug form of metronidazole → (single e- transfers) → nitrous free-radical form of metronidazole
antibiotics that affect DNA synthesis and integrity: sulfonamides
sulfonamide (sulfa) drugs act to inhibit the synthesis of folic acid
- folic acid is an important cofactor in the synthesis of nucleic acid precursors
all organisms use folic acid to synthesize nucleic acids
- bacteria make folic acid from the combination of PABA, glutamic acid, and pteridine
- mammals do not synthesize folic acid and must get it from the diet or microbes
PABA: para-aminobenzoic acid
SFA: sulfanilamide
normal folic acid formation versus formation blocked by sulfanilamide (SFA)
sulfanilamide (SFA) is a type of sulfonamide drug
- inhibits the synthesis of folic acid
normal folic acid formation
- P, PABA, G all enter the enzyme and synthesis occurs → folic acid
folic acid formation blocked by sulfanilamide (SFA)
- P, SFA, G all enter the enzyme → no synthesis occurs → P, SFA, G all leave the enzyme
SFA enters the enzyme in place of PABA
- PABA is needed for folic acid synthesis
antibiotics that affect DNA synthesis and integrity: quinolones
DNA gyrase bound to and inactivated by a quinolone will block progression of a DNA replication fork
bacterial DNA gyrases are structurally distinct from their mammalian counterparts…
- quinolone antibiotics will not affect mammalian DNA replication
RNA synthesis inhibiting antibiotics: rifampin (rifamycin)
rifampin is the best known member of the rifamycin family of antibiotics
- selectively binds to bacterial RNA polymerase and prevents transcription
rifampin is also used to treat tuberculosis and meningococcal meningitis
antibiotics that inhibit protein synthesis: mode of action
the major classes of protein synthesis inhibitors target the 30S or 50S subunits of cytoplasmic ribosomes
antibiotics that inhibit protein synthesis: drugs that affect the 30S ribosomal subunit
- aminoglycosides
- streptomycin, gentamicin, tobramycin - tetracyclines
- doxycycline - glycylcyclines
- tigecycline
antibiotics that inhibit protein synthesis: aminoglycosides
causes misreading of mRNA and inhibits peptidyl-tRNA translocation
aminoglycosides in particular:
- streptomycin
- gentamicin
- tobramycin
affects the 30S ribosomal subunit
antibiotics that inhibit protein synthesis: tetracyclines
binds to the 30S subunit and prevent tRNAs carrying amino acids from entering the A site
tetracycline in particular:
- doxycycline
affects the 30S ribosomal subunit
antibiotics that inhibit protein synthesis: glycylcyclines
binds to the 30S subunit and inhibits the entry of aminoacyl-tRNA into the A site
- it is able to function in tetracycline resistant cells
glycylcycline in particular:
- tigecycline
affects the 30S ribosomal subunit
antibiotics that inhibit protein synthesis: drugs that affect the 50S ribosomal subunit
- chloramphenicol
- macrolides
- erythromycin
- azithromycin
- clarithromycin - lincosamides
- clindamycin - oxazolidinones
- streptogramins
- quinupristin, dalfopristin
antibiotics that inhibit protein synthesis: chloramphenicol
prevents peptide bond formation
- done by inhibiting peptidyltransferase in the 50S subunit
affects the 50S ribosomal subunit
antibiotics that inhibit protein synthesis: macrolides
binds to 50S subunit and inhibit translocation of tRNA from the A site to the P site
further broken down into:
- erythromycin
- azithromycin
- clarithromycin
affects the 50S subunit of ribosomes
antibiotics that inhibit protein synthesis: lincosamides
binds to peptidyltransferase and prevents peptide bond formation from the ribosome
further broken down into:
- clindamycin
affects the 50S subunit of ribosomes
antibiotics that inhibit protein synthesis: oxazolidinones
binds to 50S subunit and prevent assembly of the 70S ribosome
affects the 50S subunit of ribosomes
antibiotics that inhibit protein synthesis: streptogramins
bind to 50S subunit and block tRNA entry into the A site
- while blocking exit of a growing protein from the ribosome
further broken down into:
- quinupristin
- dalfopristin
affects the 50S subunit of ribosomes
mechanisms of drug resistance
- drug modification or inactivation
- blocked penetration
- efflux pumps
- target modification
- target overproduction
- enzymic bypass
- target mimicry
mechanisms of drug resistance: drug modification or inactivation
enzymes can be coded by genes that can chemically modify an antimicrobial
- which inactivates it
- or can destroy an antimicrobial through hydrolysis
inactivation of enzymes can affect:
- β-lactams
- aminoglycosides
- macrolides
- rifamycins
mechanisms of drug resistance: blocked penetration
altering porins in the outer membrane
blocked penetration can affect:
- β-lactams
- tetracyclines
- fluoroquinolones
e.g. (for understanding only)
- a common mechanism of carbapenem resistance among P. aeruginosa is to decrease amount of OprD porin
- which is the primary portal of entry for carbapenems
mechanisms of drug resistance: efflux pump
transport proteins that allow microorganisms to regulate their internal environment by removing toxic substances
- can export multiple antimicrobial drugs
- prevents the accumulation of drug to a level that would be antibacterial
efflux pump can affect:
- fluoroquinolones
- aminoglycosides
- tetracyclines
- β-lactams
- macrolides
mechanisms of drug resistance: target modification
antimicrobial drugs have very specific targets
- structural changes to those targets can prevent drug binding
- this renders the drug ineffective
mechanism allows a formerly inhibited reaction to occur
- e.g. for understanding only: penicillin-binding proteins (PBPs) can inhibit the binding of β-lactam drugs and provide resistance to multiple drugs within this class
target modification can affect:
- fluoroquinolones
- rifamycins
- vancomycin
- β-lactams
- macrolides
- aminoglycosides
mechanisms of drug resistance: target overproduction
microbe overproduces the target enzyme
- such that there is a sufficient amount of antimicrobial-free enzyme to carry out the proper enzymatic reaction
mechanisms of drug resistance: enzymatic bypass
microbe develops a bypass that circumvents the need for the functional target enzyme
blocks the pathway with the functional target enzyme
- alternate route is used
mechanisms of drug resistance: target mimicry
production of proteins that bind and sequester drugs
- preventing the drugs from binding to their bacterial cellular target
fighting drug resistance
dummy target compounds that inactivate resistance enzymes have been developed
e.g.
- Clavulanic acid binds and ties up β-lactamases secreted from penicillin resistant bacteria
individuals can take the following actions to help fight drug resistance
- frequent hand washing
- vaccinations
- avoiding use of antibiotics for viral infections
- refusing leftover antibiotics
- take full course of antibiotics prescribed
fighting drug resistance: effects of taking full course of antibiotics prescribed
day 0:
- highly sensitive organisms: moderate amount
- intermediate organisms: large amount
- highly resistant organisms: moderate amount
→ graph resembles a bell curve
day 3:
- highly sensitive organisms: low amount
- intermediate organisms: moderate amount
- highly resistant organisms: moderate amount
day 6:
- highly sensitive organisms: low amount
- intermediate organisms: low amount
- highly resistant organisms: moderate amount
day 10 (antibiotics are stopped prematurely):
- highly sensitive organisms: low amount
- intermediate organisms: low amount
- highly resistant organisms: high amount
→ relapse with resistant organisms
→ resistant organisms can also spread to other hosts, causing more drug-resistant infections
day 10 (full course of antibiotic treatment finished):
- low amount of all types of organisms
- end of successful treatment
components of the influenza virus
hemagglutinin and neuraminidase
- influenza virus must be treated with antiviral agents that prevent virus uncoating or release
hemagglutinin
binds to the host membrane receptors for entry by phagocytosis
- component of the influenza virus
neuraminidase
cleaves sialic acid to allow virus particles to escape from infected cells
- component of the influenza virus
antiviral agents that prevent virus uncoating or release
- amantadine
- oseltamivir (Tamiflu)
- zanamivir (Relenza)
antiviral agents: amantadine
prevents the virus from uncoating and exiting by changing the pH of the phagolysosome
antiviral agents: oseltamivir (Tamiflu) & zanamivir (Relenza)
neuraminidase inhibitors prevent the virus particles from leaving the cell
antiviral DNA and RNA synthesis inhibitors
- zidovudine
- acyclovir
- deoxyribonucleotide containing thymine
- deoxyguanosine
antiviral agents: types of nucleoside and nonnucleoside reverse transcriptase inhibitors
- protease inhibitors
- Nelfinavir (Viracept)
- Iopinavir (Kaletra) - entry inhibitors
- CCR5 inhibitors (maraviroc)
nucleoside and nonnucleoside reverse transcriptase inhibitors: protease inhibitors
targets the HIV protease enzyme
- Nelfinavir (Viracept)
- Iopinavir (Kaletra)
nucleoside and nonnucleoside reverse transcriptase inhibitors: entry inhibitors
block virus envelope protein gp120 from binding to the host receptor CCR5
- CCR5 inhibitors (maraviroc)
antifungal agents
- polyenes
- nystatin, amphotericin B - azoles
- imidazoles, triazoles - allylamines
- terbinafine, lamisil - echinocandins
- caspofungin - griseofulvin
- flucytosine
antifungal agents: polyenes
disrupts membrane integrity
- nystatin
- amphotericin B
antifungal agents: azoles
interferes with ergosterol synthesis
- imidazoles
- triazoles
antifungal agents: allylamines
interferes with ergosterol synthesis
- terbinafine
- lamisil
antifungal agents: echinocandins
blocks fungal cell wall synthesis
- caspofungin
antifungal agents: griseofulvin
blocks cell division
antifungal agents: flucytosine
inhibits DNA synthesis
antiparasitic agents include
- antiprotozoan agents
- antihelminthic agents
antiprotozoan agents
most are fairly toxic
- metronidazole
- quinine
- chloroquinine, primaquine (antimalarials)
antiprotozoan agents: metronidazole
causes DNA breakage
- used to treat giardisis and Trichomonas infections
antiprotozoan agents: quinine
commonly used in the past
- now used as a last resort to treat malaria
antiprotozoan agents: chloroquinine & primaquine
interfere with protein synthesis
- specially RBCs
- these are antimalarials
antihelminthic agents
- niclosamide
- tapeworms - praziquantel
- flatworms - mebendazole and albendazole
- intestinal roundworms - ivermectin
- intestinal roundworms
antihelminthic agents: niclosamide
prevents ATP generation
- targets tapeworms
antihelminthic agents: praziquantel
alters membrane permeability
- targets flatworms
antihelminthic agents: mebendazole and albendazole
interferes with nutrient absorption
- targets intestinal roundworms
antihelminthic agents: ivermectin
causes paralysis of helminths
- targets intestinal roundworms
