Micro POM Flashcards
Major targets for antibacterials
- Cell wall synthesis
- Membrane structure
- Folate synthesis
- DNA synthesis
- Protein synthesis
Targets that are unique to microbes (humans don’t have)
- Cell envelope
- Prokaryotic ribosome
- Prokaryotic nucleic acid metabolism
- Essential nutrient synthesis
Minimum Inhibitory Concentration:
The smallest amount of drug needed to INHIBIT the growth of a particular bacterial species (smaller value than MBC)
Minimum Bactericidal Concentration:
The smallest amount of drug needed to KILL a particular bacterial species (larger value than MIC)
Bacteriostatic:
Bacteria are stopped from growing but aren’t killed.
e.g. tetracycline
Bacteriocidal:
Bacteria are killed.
e.g. penicillin
Methods to determine microbial susceptibility/resistance:
- Culture-based (E-test and Disk diffusion e.g. A disk)
- Molecular detection of resistance mutations (PCR, sequencing, etc.)
Hospital antibiograms:
- Periodic summaries of antibiotic susceptibilities of local isolates sent to clinical micro lab (results are % of strains tested that are susceptible)
- Aid in selecting empiric therapy
- Track resistance trends
The efficacy of antimicrobial drugs is limited by:
- Mechanism of action
- Susceptibility of the target organism
- Side effects on the host
- Pharmacodynamics
- Cost
- Patient compliance
Cmax
The max concentration that can be achieved from a given dose of a drug
-always the peak of the curve
Area Under Curve (AUC)
Area under the curve tells the total amount of drug that has accumulated in the patient. Want to maximize the amount of time that this is above the MIC level.
Time-dependent killing (TDK)
GOAL: Maximize time above MIC
Drug > MIC for at least 50% of dosing interval
e.g. penicillins and cephalosporins
Concentration-dependent killing (CDK)
GOAL: Maximize Cmax and therefore AUC
AUC/MIC > 30 for G+
AUC/MIC > 130 for G-
Post-antibiotic effect (PAE):
The time it takes bacteria to return to log-phase growth following removal of antibiotic
(usually minimal for TDK because already tried to extend the time the drug is above the MIC; CDK usually has a longer PAE)
______ PAEs reduce required frequency of dosing, and reduce toxicities and costs
LONGER PAEs reduce required frequency of dosing, and reduce toxicities and costs
Classes of agents that interfere with the bacterial cell envelope:
- B-lactams
- Glycopeptides
- Isoniazid
- Ethambutol
- Bacitracin
- Phospho(no)mycin
- Cycloserine
- Lipopeptides
- Polymyxins
Structures unique to gram negative cells:
Outer membrane beyond the peptidoglycan, composed of LPS (lipopolysaccharide) and Lipid A (the toxic molecule of LPS)
Structures unique to gram positive cells:
Lipoteichoic acid (LTA) within peptidoglycan
General structure of peptidoglycan:
Alternating NAM and NAG sugars make the basic backbone chain of peptidoglycan (connected via transglycosylation)
On the NAM molecules, we see an extension of 4 amino acids that can be used to create the cross-linking and provide rigidity. (connected via transpeptidation)
What are the last two amino acids in the peptide chains of peptidoglycan?
D-Ala-D-Ala
Penicillin Binding Proteins (PBP)
PBPs have transpeptidase activity (some also have transglycosylase activity)
Where do penicillins and cephalosporins target?
These target the transpeptidation reactions that are essential for bacterial synthesis
Antibiotic resistance mechanisms:
- Enzymatically inactivate drug (B-lactamases) - often on mobile genetic elements and can be transferred between bacteria
- Alter drug target (mutation, can occur via horizontal exchange)
- Alter drug exposure (decreased uptake in G-, increase efflux)
B-lactamases:
Break a bond in the B-lactam ring of penicillin to disable the molecule. Bacteria with this enzyme can resist the effects of penicillin and other B-lactam antibiotics.
2 Important B-lactamases:
- ESBL (extended spectrum B-lactamases): mostly derived from active site mutations in TEM/SHV; results in activity against extended-spectrum cephalosporins
- Metal-Dependent (New Delhi Metallo B-lactamase): NDM-1
Clavulanic Acid:
Category of drugs that were developed as B-lactamase inhibitors. Deactivate the microbial enzyme B-lactamase.
Alternative Penicillin-Resistant PBPs:
- Some PBPs have low affinities from B-lactams but still retain transpeptidase activity
- Can arise through mutation (e.g. gonorrhea)
- Can be acquired horizontally (e.g. mecA gene in MRSA)
Antibiotic resistance often exacts a ______ cost.
Antibiotic resistance often exacts a FITNESS cost (The resistant bacteria don’t grow as well because their PBOs aren’t as good at making peptidoglycan as the original susceptible version of the enzyme).
Gram negative cell bacterial resistance:
- Outer membrane helps keep things out. Also have porin proteins in membrane that can both prevent entry.
- B-lactamases located between the inner and outer membranes
- Can acquire an efflux pump that allows them to quickly push out antibiotics
Glycopeptide drug class:
VANCOMYCIN
Inhibit the transglycosylation of peptidoglycan.
Vancomycin
Binds to the D-Ala-D-Ala residues, which normally function to direct transglycosylation sugar linkages. By binding, vancomycin prevents this by directly covering the actual peptide residues.
Vancomycin resistance mechanisms:
- The D-Ala-D-Ala target of glycopeptides is not encoded by a gene (so spontaneous mutation can’t confer resistance)
- No known enzymes capable of inactivating glycopeptide antibiotics
- Vancomycin is primarily used to treat Gram positive infections
- Target is extracellular precluding altered drug uptake as a resistance mechanism
D-ala-D-lac
Resistance mechanism developed by some bacteria that mutates a D-Ala residue, thus preventing Vancomycin from binding and blocking transglycosylation. BUT this peptidoglycan isn’t as strong, and makes the bacteria more susceptible to a lot of other things.
Bacitracin:
- Cell-wall active agent
- Inhibits regeneration of peptidoglycan lipid carrier
Phosphomycin:
- Cell wall active agent
- Prevents attachment of NAG to NAM + peptide
Cycloserine:
- Cell wall active agent
- Prevents attachment of peptide to NAM
Mycobacterium species:
M. tuberculosis and M. leprae
- Mycolic acid: waxy long-chain branched hydrocarbons
- Arabinogalactan (sugar-like molecule that adds to the structural integrity)
- Acid-fast stain, not gram stain
Agents that act on Mycobacterial cell walls:
- Isoniazid
- Ethambutol
Isoniazid:
Inhibits mycolic acid synthesis
Ethambutol:
Thought to inhibit arabinotransferases
Lipopeptides:
Disrupt the cell membrane of G+ bacteria.
- Form pores in cytoplasmic membrane
- Too big to get through porins in Gram- outer membrane
- Bind to phosphatidylglycerol, which is an abundant component of bacterial cell membranes but rare in eukaryotic cells
Lipopeptide example:
Daptomycin
________ can NOT be used to treat pneumonia.
LIPOPEPTIDES can NOT be used to treat pneumonia, because the surfactant found in human lungs is rich in phosphatidylglycerol
Bacterial folate synthesis inhibitors:
- Sulfonamides
- Trimethoprim
Sulfonamides:
-Bacteriostatic on their own, active against G+ and G-
-Many bacteria synthesize their own folate (humans don’t - thus, selectivity)
E.g. SULFAMETHOXAZOLE
Resistance to sulfonamides:
- Altered drug target (spontaneous mutation in dhps gene), horizontal acquisition of alternate DHPS encoded on mobile genetic elemenet
- Swamp the system (increased production of folate precursor PABA)
- Altered drug exposure (decreased uptake)
Trimethoprim inhibits DHFR
- bactericidal, used in combo with sulfamethoxazole
- Bacterial DHFR is much more sensitive to drug than human enzyme
- tmp/smx combo is synergistic - smx becomes bactericidal using lower doses of both drugs
- targeting 2 separate steps in the same pathway reduces likelihood of resistance
Quinolones/Fluoroquinolones inhibit prokaryotic _____ synthesis
Quinolones/Fluoroquinolones inhibit prokaryotic DNA synthesis.
- Bactericidal (G- better than G+)
- Inhibit DNA gyrase (aka topoisomerase II) and topoisomerase IV
e. g. ciprofloxacin
What class of antibiotics is ciprofloxacin in?
Quinolones/Fluoroquinolones
Fluoroquinolones inhibit _______ and _______
Fluoroquinolones inhibit Gyrase (topoisomerase II), inducing damage during DNA replication, and Topoisomerase IV, preventing newly-replicated chromosomes from separating into daughter cells.
_________ should be reserved for patients who have no other treatment options due to its black box warning related to tendon rupture.
FLUOROQUINOLONES should be reserved for patients who have no other treatment options due to its black box warning related to tendon rupture.
Resistance to quinolones:
- Altered Drug Target (chromosomal mutations in gyrase and topoisomerase genes)
- Altered Drug Exposure (decreased uptake via mutations in gram negative porin proteins, increased efflux due to mutations that increase efflux pump activity, cross-resistance between quinolones and other antibiotics and host-derived antimicrobial factors = multi drug resistance)
Rifamycins inhibit _____ synthesis:
Rifamycins inhibit mRNA synthesis:
-Can be bactericidal or bacteriostatic depending on concentration
-primarily used for tuberculosis or meningococcal prophylaxis
-Bind to bacterial DNA-dependent RNA polymerase with higher affinity than to human enzyme
E.g. rifampin
Resistance to rifamycins:
- Rarely used as monotherapy because of high rate of resistance
- Resistance can occur with spontaneous mutations in RNA polymerase gene (this mutation has a very low fitness cost so it happens fast)
Nitroimidazoles damage _______.
Nitroimidazoles damage DNA (by oxidative mechanisms)
- Are taken in inactive form and must be converted by microbial enzyme ferredoxin to active form (ferredoxin removes nitro group).
- Activated drug forms free radicals that damage DNA
e. g. metronidazole
Resistance to nitroimidazoles:
Failure to enzymatically activate drug (mutations in microbial enzymes that convert the prodrug to the active compound)
Name the subclasses of B-lactam antibiotics (4):
Penicillins, cephalospirins, carbapanems, and monobactams
