Antimicrobial Agents & Microbial Resistance Flashcards
What are antimicrobial agents?
Have activity against microbes; antibacterials, antibiotics (produced by a microbe)
Antibacterial mechanisms of action
- cell wall synthesis
- membrane structure
- DNA synthesis
- Folate synthesis
- DNA replication
- Protein synthesis
Minimum Inhibitory Concentration (MIC) vs Minimum Bactericidal Concentration (MBC)
MIC: minimum to inhibit growth
MBC: minimum to kill bug
Bacteriostatic vs. Bactericidal
Bacteriostatic, MBC ≫ MIC
Bactericidal, MBC ≊ MIC
Methods to determine microbial susceptibility/resistance
Culture-based
- Disk diffusion
- E-test
Molecular detection of resistance mutations
- PCR/sequencing
Efficacy of antimicrobial drugs is limited by:
- Mechanism of action
- Susceptibility of the target organism
- Side effects on the host
- Pharmacodynamics
- Cost
- Patient compliance
Time-dependent killing (TDK)
Goal is to maximize time above MIC; Serum [drug] > MIC for at least 50% of dosing interval
Time-dependent killing (TDK) Examples
Wall inhibitors: Penicillins, Cephalosporins
Protein inhibitors: Macrolides, Clindamycin
Concentration-dependent killing (CDK)
GOAL: Maximize Cmax and therefore AUC.
AUC/MIC > 30 for G+
AUC/MIC > 130 for G-
Concentration-dependent killing (CDK) Examples
DNA inhibitor: Fluoroquinolones
Protein inhibitor: Aminoglycosides
Post-antibiotic effect (PAE)
The time it takes bacteria to return to log-phase growth following removal of antibiotic
TDK: minimal PAE
CDK: prolonged PAE - reduced frequency of dosing/toxicity/cost
Bacterial cell envelope
- Cell membrane (a.k.a. inner or plasma membrane)
- Peptidoglycan layer
- Outer membrane (for Gram-negatives)
Gram -/+ cell envelope
Gram (-): lipopolysaccharide + lipid A + small peptidoglycan
Gram (+): larger peptidoglycan + lipoteichoic acid
Peptidoglycan
N-acetylmuramic acid (NAM) + N-acetylglucosamine (NAG) with cross-linked peptides
β-lactams
bind Penicillin Binding Proteins (PBPs) and prevent transpeptidation; structurally similar to D-Ala-D-Ala
Examples: penicillins and cephalosporins
Antibiotic Resistance Mechanisms
- Enzymatically Inactivate Drug (β-lactamases)
- Alter Drug Target
- Alter Drug Exposure
β-lactamases
Breaks bond in β-lactam ring of penicillin to disable molecule; penicillin resistance
β-lactamase classifications (2)
- ESBL (extended spectrum β-lactamases)
- Mostly derived from active site mutations in TEM/SHV, activity against extended-spectrum cephalosporins
- Metal-dependent/New Delhi Metallo-β-lactamase
- NDM-1
Clavulanic Acid
Inhibits β-lactamases
Alternative Penicillin-Resistant PBPs
Some have low affinities for β-lactams but retain transpeptidase activity; can be aquired through mutation or horizontally (MRSA)
Selective pressure and reversibility of resistance
Antibiotic resistance often exacts a fitness cost; additional mutations can compensate for fitness cost; low fitness cost = less reversible
Choose antibiotics with higher fitness cost for resistance
Altered Penicillin Transport
- Decreased membrane permeability (gram -); spontaneous mutations in porin genes
- Incrased efflux; horiztontal aquisition of new pump/mutation that alters specificity or expression
Glycopeptides
Inhibit transglycosylation of peptidoglycan (Ex. Vancomycin)
Glycopeptide resistance
Synthesizes D-Ala-D-Lac, which vancomycin cannot bind and therefore cannot prevent from incorporation into peptidoglycan chain
What characterizes mycobacterium?
Mycolic acid: waxy long-chain branched hydrocarbons; requires acid-fast stain instead of gram stain
Drugs that act on mycobacterial cell walls
- Isoniazid (inhibits mycolic acid synthesis)
- Ethambutol (inhibits arabinotransferases)
Lipopeptides
Disrupt cell membrane of gram + bacteria
Form pores in cytoplasmic membrane; bind to phosphatidyl-glycerol (lots in bacterial, rare in eukarotic, cell membranes)
What are lipopeptides not used to treat?
Pneumonia; lung surfactant is rich in phospatidyl-glycerol
Bacterial folate synthesis inhibitors
- Sulfonamides
- Trimethoprim
Sulfonamides + example
Inhibit folate precursor synthesis; bacteriostatic; actie against G+/-, some protozoa; selective – humans don’t synthesize own folate
Example: sulfamethoxazole (smx)
Resistance to sulfonamides
- Altered drug target (spontaneous mutations in dhps gene or horizontal acquisition of alternate dhps
- Swamp system with folate precursor PABA
- Altered drug exposure (decrease uptake)
Combination Therapy
- Prevent the emergence of resistance
- Treatment of emergency cases when etiology is still unknown
- Take advantage of combinatorial synergy
Trimethoprim
Inhibits DHFR
Bactericidal, used in combo with smx – synergistic
Quinolones/Fluoroquinolones + Example
inhibit prokaryotic DNA synthesis - inhibit DNA gyrase, inducing DNA damage
Bactericidal (G- > G+)
2nd gen fluoroquinolone: ciprofloxacin
Quinolone resistance
- Altered drug target (gyrase mutations)
- Altered drug exposure (decrease uptake, increase efflux, cross resistance –> multidrug resistance MDR
Rifamycins + Example
Inhibit mRNA synthesis
bactericidal or bacteriostatic depending on concentration, primarily used for Mycobacterium tuberculosis or for meningococcal prophylaxis
Bind to bacterial DNA-dependent RNA polymerase w/ higher affinity than to human enzyme
example: rifampin
Resistance to rifamycins
COMMON/QUICK
- Altered drug target – spontaneous mutation in RNA polymerase gene
Nitroimidazoles + Example
Damage DNA; Pro-drugs; Activated drugs form free radicals
Bactericidal vs. anaerobic microbes including bacteria and some protozoa
Example: Metronidazole
Resistance to nitroimidazoles
- Failure to enzymatically activate drug
