Antimicrobial Agents & Microbial Resistance

Question 1

What are antimicrobial agents?

Answer 1

Have activity against microbes; antibacterials, antibiotics (produced by a microbe)

Question 2

Antibacterial mechanisms of action

Answer 2

1. cell wall synthesis
2. membrane structure
3. DNA synthesis
   
   • Folate synthesis
   • DNA replication
4. Protein synthesis

Question 3

Minimum Inhibitory Concentration (MIC) vs Minimum Bactericidal Concentration (MBC)

Answer 3

MIC: minimum to inhibit growth

MBC: minimum to kill bug

Question 4

Bacteriostatic vs. Bactericidal

Answer 4

Bacteriostatic, MBC ≫ MIC

Bactericidal, MBC ≊ MIC

Question 5

Methods to determine microbial susceptibility/resistance

Answer 5

Culture-based

1. Disk diffusion
2. E-test

Molecular detection of resistance mutations

1. PCR/sequencing

Question 6

Efficacy of antimicrobial drugs is limited by:

Answer 6

1. Mechanism of action
2. Susceptibility of the target organism
3. Side effects on the host
4. Pharmacodynamics
5. Cost
6. Patient compliance

Question 7

Time-dependent killing (TDK)

Answer 7

Goal is to maximize time above MIC; Serum [drug] > MIC for at least 50% of dosing interval

Question 8

Time-dependent killing (TDK) Examples

Answer 8

Wall inhibitors: Penicillins, Cephalosporins

Protein inhibitors: Macrolides, Clindamycin

Question 9

Concentration-dependent killing (CDK)

Answer 9

GOAL: Maximize Cmax and therefore AUC.

AUC/MIC > 30 for G+

AUC/MIC > 130 for G-

Question 10

Concentration-dependent killing (CDK) Examples

Answer 10

DNA inhibitor: Fluoroquinolones

Protein inhibitor: Aminoglycosides

Question 11

Post-antibiotic effect (PAE)

Answer 11

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

Question 12

Bacterial cell envelope

Answer 12

1. Cell membrane (a.k.a. inner or plasma membrane)
2. Peptidoglycan layer
3. Outer membrane (for Gram-negatives)

Question 13

Gram -/+ cell envelope

Answer 13

Gram (-): lipopolysaccharide + lipid A + small peptidoglycan

Gram (+): larger peptidoglycan + lipoteichoic acid

Question 14

Peptidoglycan

Answer 14

N-acetylmuramic acid (NAM) + N-acetylglucosamine (NAG) with cross-linked peptides

Question 15

β-lactams

Answer 15

bind Penicillin Binding Proteins (PBPs) and prevent transpeptidation; structurally similar to D-Ala-D-Ala

Examples: penicillins and cephalosporins

Question 16

Antibiotic Resistance Mechanisms

Answer 16

1. Enzymatically Inactivate Drug (β-lactamases)
2. Alter Drug Target
3. Alter Drug Exposure

Question 17

β-lactamases

Answer 17

Breaks bond in β-lactam ring of penicillin to disable molecule; penicillin resistance

Question 18

β-lactamase classifications (2)

Answer 18

1. ESBL (extended spectrum β-lactamases)
   
   • Mostly derived from active site mutations in TEM/SHV, activity against extended-spectrum cephalosporins
2. Metal-dependent/New Delhi Metallo-β-lactamase
   
   • NDM-1

Question 19

Clavulanic Acid

Answer 19

Inhibits β-lactamases

Question 20

Alternative Penicillin-Resistant PBPs

Answer 20

Some have low affinities for β-lactams but retain transpeptidase activity; can be aquired through mutation or horizontally (MRSA)

Question 21

Selective pressure and reversibility of resistance

Answer 21

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

Question 22

Altered Penicillin Transport

Answer 22

1. Decreased membrane permeability (gram -); spontaneous mutations in porin genes
2. Incrased efflux; horiztontal aquisition of new pump/mutation that alters specificity or expression

Question 23

Glycopeptides

Answer 23

Inhibit transglycosylation of peptidoglycan (Ex. Vancomycin)

Question 24

Glycopeptide resistance

Answer 24

Synthesizes D-Ala-D-Lac, which vancomycin cannot bind and therefore cannot prevent from incorporation into peptidoglycan chain

Question 25

What characterizes mycobacterium?

Answer 25

Mycolic acid: waxy long-chain branched hydrocarbons; requires acid-fast stain instead of gram stain

Question 26

Drugs that act on mycobacterial cell walls

Answer 26

1. Isoniazid (inhibits mycolic acid synthesis) 2. Ethambutol (inhibits arabinotransferases)

Question 27

Lipopeptides

Answer 27

Disrupt cell membrane of gram + bacteria Form pores in cytoplasmic membrane; bind to phosphatidyl-glycerol (lots in bacterial, rare in eukarotic, cell membranes)

Question 28

What are lipopeptides not used to treat?

Answer 28

Pneumonia; lung surfactant is rich in phospatidyl-glycerol

Question 29

Bacterial folate synthesis inhibitors

Answer 29

1. Sulfonamides 2. Trimethoprim

Question 30

Sulfonamides + example

Answer 30

Inhibit folate precursor synthesis; bacteriostatic; actie against G+/-, some protozoa; selective -- humans don't synthesize own folate Example: sulfamethoxazole (smx)

Question 31

Resistance to sulfonamides

Answer 31

1. Altered drug target (spontaneous mutations in dhps gene or horizontal acquisition of alternate dhps 2. Swamp system with folate precursor PABA 3. Altered drug exposure (decrease uptake)

Question 32

Combination Therapy

Answer 32

1. Prevent the emergence of resistance 2. Treatment of emergency cases when etiology is still unknown 3. Take advantage of combinatorial synergy

Question 33

Trimethoprim

Answer 33

Inhibits DHFR Bactericidal, used in combo with smx -- synergistic

Question 34

Quinolones/Fluoroquinolones + Example

Answer 34

inhibit prokaryotic DNA synthesis - inhibit DNA gyrase, inducing DNA damage Bactericidal (G- \> G+) 2nd gen fluoroquinolone: ciprofloxacin

Question 35

Quinolone resistance

Answer 35

1. Altered drug target (gyrase mutations) 2. Altered drug exposure (decrease uptake, increase efflux, cross resistance --\> multidrug resistance MDR

Question 36

Rifamycins + Example

Answer 36

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

Question 37

Resistance to rifamycins

Answer 37

COMMON/QUICK 1. Altered drug target -- spontaneous mutation in RNA polymerase gene

Question 38

Nitroimidazoles + Example

Answer 38

Damage DNA; Pro-drugs; Activated drugs form free radicals Bactericidal vs. anaerobic microbes including bacteria and some protozoa Example: Metronidazole

Question 39

Resistance to nitroimidazoles

Answer 39

1. Failure to enzymatically activate drug