Microbiology Cheat Sheet
Resistance & Susceptibility
Antimicrobial resistance and susceptibility testing for NEET-PG: MRSA mecA, ESBL, VRE, carbapenemases, Kirby-Bauer, MIC, and the D-zone test.
MedNext Academy | 3 min read
Resistance & Susceptibility
Antimicrobial resistance and susceptibility testing for NEET-PG: MRSA mecA, ESBL, VRE, carbapenemases, Kirby-Bauer, MIC, and the D-zone test.
This topic covers the mechanisms by which bacteria resist antimicrobials, how resistance genes spread, and the laboratory methods used to test antibiotic susceptibility.
High-yield lines
- MRSA carries the mecA gene encoding PBP2a, which has low affinity for all beta-lactams including penicillins, cephalosporins, and carbapenems.
- ESBL-producing Enterobacterales are resistant to third-generation cephalosporins but susceptible to carbapenems, which are the drugs of choice.
- The double-disc synergy test with clavulanate confirms extended-spectrum beta-lactamase production by zone enhancement of at least 5 millimetres.
- Vancomycin resistance in enterococci (VRE) results from vanA or vanB genes that substitute D-Ala-D-Lac for the D-Ala-D-Ala vancomycin target.
- Conjugation, the direct transfer of plasmid DNA via a sex pilus, is the most clinically important mechanism for spreading resistance genes.
- Carbapenem-resistant Enterobacterales produce KPC, NDM, or OXA-48 carbapenemases, and India is an epicentre for NDM-1-producing Klebsiella.
- The Kirby-Bauer disc diffusion method uses a 0.5 McFarland inoculum on Mueller-Hinton agar and interprets zone diameters against CLSI or EUCAST breakpoints.
- Mueller-Hinton agar is the standard AST medium because it is non-selective, low in sulphonamide antagonists, and has reproducible cation content.
- The minimum inhibitory concentration (MIC) is the lowest antibiotic concentration inhibiting visible growth, with broth microdilution as the gold standard.
- The D-zone test detects inducible clindamycin resistance in staphylococci and streptococci, seen as flattening of the clindamycin zone next to an erythromycin disc.
- A cefoxitin disc zone of 21 millimetres or less indicates mecA-mediated methicillin resistance as a surrogate for oxacillin susceptibility.
- Aminoglycoside efficacy is predicted by peak levels (concentration-dependent killing) while trough levels predict nephrotoxicity and ototoxicity.
- Efflux pumps such as MexAB-OprM in Pseudomonas actively extrude antibiotics and contribute to multidrug resistance.
- ESBL producers characteristically remain susceptible to cephamycins like cefoxitin; if cefoxitin resistance is also present, suspect AmpC beta-lactamase.
Mapped competency codes
- MI1.6
Continue into the full chapter
This summary maps to MI1-general-microbiology-and-immunity.
Frequently Asked Questions
Why is MRSA resistant to all beta-lactams?
The mecA gene encodes PBP2a, an altered penicillin-binding protein with uniformly low affinity for the entire beta-lactam class.
What is the treatment of choice for ESBL-producing Enterobacterales?
Carbapenems, because ESBL producers are resistant to third-generation cephalosporins but retain carbapenem susceptibility.
Which medium is standard for disc diffusion susceptibility testing?
Mueller-Hinton agar, chosen for being non-selective, low in trimethoprim and sulphonamide antagonists, and reproducible in cation content.
What does the D-zone test detect?
Inducible clindamycin (iMLSB) resistance, shown by flattening of the clindamycin inhibition zone adjacent to an erythromycin disc.
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