Microbiology
Microbiology tests specimen quality, culture workup, organism identification, and susceptibility reporting. The exam emphasizes why an unacceptable specimen cannot be rescued by good technique, and how a Gram stain plus a few key biochemicals narrows an identification.
Specimen quality and transport
Sputum is screened on Gram stain: acceptable when there are fewer than 10 squamous epithelial cells and more than 25 polymorphonuclear cells per low-power field. A specimen that fails this is saliva and should be rejected, not reported. Urine for culture should reach the laboratory within 2 hours or be refrigerated; delay permits overgrowth and false-positive interpretation. Blood cultures are drawn before antibiotics, two sets from separate sites, with an adequate volume per bottle. Anaerobic specimens require anaerobic transport and must never be put on a swab in air.
Media and identification
Blood agar is non-selective and shows hemolysis. Chocolate agar supports fastidious organisms such as Haemophilus and Neisseria. MacConkey agar selects gram-negative rods and detects lactose fermentation. Columbia CNA selects gram-positive organisms. Thioglycollate broth supports anaerobes. Catalase and coagulase separate Staphylococcus aureus (coagulase positive) from coagulase-negative staphylococci. Among streptococci, bacitracin sensitivity suggests group A, optochin sensitivity and bile solubility suggest Streptococcus pneumoniae, and bile esculin suggests group D. Oxidase positivity fits Neisseria and Pseudomonas. Lactose fermentation on MacConkey produces pink colonies, typical of E. coli.
Susceptibility testing
Disk diffusion is standardized by CLSI with a 0.5 McFarland inoculum and defined incubation. MIC methods (broth microdilution, gradient diffusion) give quantitative results. Report only validated panels, and never report intrinsic resistance as susceptible, for example Klebsiella against ampicillin.
Worked example
A wound Gram stain shows gram-positive cocci in clusters. Blood agar grows beta-hemolytic colonies that are catalase positive and coagulase positive, identifying Staphylococcus aureus. Susceptibility is reported, with cefoxitin or oxacillin resistance indicating MRSA and requiring alternative therapy.
Traps that cost marks
- Calling a sputum Gram stain positive from a specimen full of squamous cells.
- Reporting mixed normal flora as a pathogen.
- Reading plates before 18-24 hours of incubation.
- Misidentifying Enterococcus as a Streptococcus.
- Ignoring an anaerobic culture request when the source is deep or abscessed.