Microbiology
Microbiology items test specimen quality, staining, culture and identification logic, and susceptibility interpretation. The exam rewards a stepwise approach: is the specimen acceptable, what does the Gram stain show, what does the colony and biochemical pattern identify, and what does the susceptibility result mean for therapy.
Specimen and stain
- Reject specimens with quality problems: sputum with many squamous epithelial cells, unlabeled containers, delayed transport, or anaerobic culture requested from a site that cannot be collected anaerobically.
- Gram stain reports shape, arrangement, and reaction (for example, gram-positive cocci in clusters). It guides early therapy and culture interpretation.
- Blood culture yield depends mainly on volume; collect sets from separate sites before antibiotics.
Media and identification
- Blood agar grows most organisms and shows hemolysis; chocolate agar supplies factors for Haemophilus and Neisseria; MacConkey selects gram-negative bacilli and shows lactose fermentation; colistin-nalidixic acid agar selects gram positives; thioglycollate broth supports anaerobes.
- Key reactions: catalase and coagulase for staphylococci; optochin sensitivity and bile solubility for Streptococcus pneumoniae; bacitracin sensitivity and PYR for Streptococcus pyogenes; bile esculin and salt tolerance for enterococci; oxidase for pseudomonads and Neisseria.
- Enteric identification uses triple sugar iron, citrate, urease, indole and motility patterns; serotyping and MALDI-TOF confirm.
Susceptibility
- MIC is the lowest concentration inhibiting visible growth; disk diffusion zones correlate inversely with MIC.
- Cefoxitin disk screening predicts mecA-mediated methicillin resistance in Staphylococcus aureus; report such isolates as resistant to all beta-lactams.
- Extended-spectrum beta-lactamase producers resist third-generation cephalosporins but remain susceptible to carbapenems; carbapenemase producers resist carbapenems.
Worked example
A wound swab grows gram-positive cocci in clusters. The isolate is catalase positive, coagulase positive, and grows on mannitol salt agar. The cefoxitin disk zone measures 15 mm (resistant breakpoint 21 mm or less). Reasoning: catalase-positive, coagulase-positive clusters identify Staphylococcus aureus; the small cefoxitin zone indicates mecA-mediated resistance, so the isolate is reported as MRSA and all beta-lactams, including cephalosporins, are reported resistant. Vancomycin remains appropriate pending clinical review.
Traps that cost marks
- Reporting a mixed flora wound culture as a pathogen without correlating with the Gram stain.
- Interpreting a zone of inhibition without checking the correct breakpoint table or the inoculum standard.
- Calling beta-hemolysis on chocolate agar, where it does not appear.
- Failing to reject a sputum specimen that is really saliva.