Microbiology
From specimen to organism
Specimen quality determines result quality: sputum must show squamous epithelial cells below 10 per low-power field, urine should be a clean-catch midstream, and tissue or fluid beats a swab. Gram stain gives the first branch point (gram reaction, morphology, arrangement), then culture media and biochemical reactions narrow the identification, confirmed by MALDI-TOF or molecular methods.
Recognition shortcuts
- Streptococci: catalase negative; bacitracin sensitive and PYR positive = group A; optochin sensitive and bile soluble = pneumococcus; hippurate positive = group B.
- Staphylococci: catalase positive; coagulase positive = S. aureus; novobiocin resistant = S. saprophyticus.
- Enterobacterales: oxidase negative, glucose fermenting, lactose fermentation on MacConkey.
- Pseudomonas aeruginosa: oxidase positive, non-lactose fermenting, grape-like odor, growth at 42 °C.
Susceptibility and resistance
Understand MIC versus disk diffusion, breakpoint categories, and mechanisms: MRSA via mecA/PBP2a, VRE via vanA/vanB, ESBLs hydrolyzing penicillins and cephalosporins but inhibited by clavulanate, and carbapenemases (KPC, NDM).
Traps
Distinguish contamination from infection - a single coagulase-negative staphylococcus blood culture usually means skin flora. Normal flora can be pathogenic in the right site. Candida in urine is often colonization. C. difficile diagnosis requires a toxigenic algorithm, not culture alone.
Worked example: a CF patient's sputum grows a gram-negative rod that is oxidase positive, grows at 42 °C, and does not ferment lactose → Pseudomonas aeruginosa. Oxidase positivity plus non-lactose fermentation is the discriminating pair most questions test.