Microbiology
Specimen and staining
Specimen quality determines result quality: sputum is acceptable for culture when it shows >25 neutrophils and <10 squamous epithelial cells per low-power field. Gram stain classifies organisms and guides preliminary therapy. Key media: sheep blood agar (non-selective), MacConkey (gram-negative), chocolate (fastidious), CNA (gram-positive), Thayer-Martin (Neisseria).
Identification
Catalase and coagulase separate staphylococci; S. aureus is coagulase positive, coagulase-negative staphylococci are not. Optochin sensitivity and bile solubility identify S. pneumoniae; bacitracin sensitivity suggests S. pyogenes; bile-esculin with 6.5% NaCl identifies Enterococcus. Oxidase-positive non-fermenters include Pseudomonas. TSI, indole, urease, and citrate differentiate Enterobacterales. MALDI-TOF and molecular methods are now routine.
Susceptibility
MIC is the lowest concentration inhibiting visible growth; disk diffusion zones correlate with breakpoints. Watch for ESBL, AmpC, carbapenemase, MRSA (mecA), and VRE. Blood cultures require two sets with adequate volume and are typically incubated 5 days.
Traps
- Confusing S. aureus with coagulase-negative staphylococci.
- Reporting a culture from a contaminated specimen.
- Overlooking intrinsic resistance (e.g., Klebsiella is intrinsically resistant to ampicillin).
Rule of thumb: identify first with a short biochemical or MALDI algorithm, then confirm susceptibility - never skip the confirmation step.