Antibiotic Resistance

Question 1

Penicillins

Answer 1

Mostly for gram-positive cocci
Aminopenicillins (ampicillin, amoxicillin) add some gram-negative coverage.
Ticarcillin, pipericillin add pseudomonal coverage.

Question 2

4th gen cephalosporins

Answer 2

Cefepime (pseudomonas)

Ceftarolime (MRSA)

Question 3

Mechanism of action of…

Aminoglycosides
Fluoroquinolones
Macrolides

Answer 3

Aminoglycosides - Inhibit protein synthesis (-cidal)
Fluoroquinolones - Inhibit DNA topoisomerase
Macrolides - Inhibit protein synthesis (-static)

Question 4

What drugs are used for intracellular pathogens?

What drugs are used for panresistant gram-pos, or gram-neg, organisms?

Answer 4

Intracellular: Tetracyclines

Panresistant gram-pos: Linezolid
Panresistant gram-neg: Colistin

Question 5

Susceptibility interpretations: S/I/R

Answer 5

S - Susceptible
I - Intermediate (use if drug concentrates, ie Urine)
R - Resistant

Question 6

E-test

Answer 6

Gradiented diffusion test; MIC is marked where growth stops.

Question 7

Beta-lactamase test

Answer 7

Paper disk test using a chromogenic cephalosporin. Normally YELLOW, turns RED with presence of beta-lactams.

Question 8

mecA

Answer 8

Gene encoding oxacillin-resistant PBP2a. Confers resistance to all beta-lactams.

Can be screened/detecting using oxacillin screening agar or cefoxitin disk. Rapid EIA also exists.

Staph Aureus

Question 9

erm

Answer 9

Inducible or constitutive macrolide/clindamycin resistance.

Detected using D-test (two paper disks of erythro/clinda, look for blunting for induction).

Question 10

vanA
vanB
vanC

Answer 10

vanA: High level transmissible vancomycin resistance. Seen in E. Faecium.
vanB: Low-level vancomycin resistance with sensitivity to Teicoplanin.
vanC: Low-level vancomycin resistance in E. Casseliflavus / Gallinarum

Question 11

ESBLs

Answer 11

Beta-lactamases, still often inhibited by beta-lactamase inhibitors (exception: ampC).

Question 12

Carbapenemases

Answer 12

KPC: #1, enterobacteriaceae
NDM-1: Metalloproteinase
VIM: Metalloproteinase
IMP: Metalloproteinase
OXA: Acinetobacter

Question 13

How are carbapenemases detected?

Answer 13

Old (Hodge test): plate lawn of control organism, drop disks, and streak patient. Look for growth of control near disks.

CarbaR test

mCIM: Incubate patient isolate with meropenem, then do a Hodge test?
eCIM: As with eCIM, but adding EDTA (metalloproteinases are susceptible)