Microbiology: Stains, Culture Interpretation, Identification, and Susceptibility
The microbiology section tests specimen quality, staining and culture interpretation, organism identification, and susceptibility testing.
Stains and media
Gram-positive organisms retain crystal violet-iodine through thick peptidoglycan and stain purple; gram-negative organisms decolorize and counterstain pink. Blood agar shows hemolysis; chocolate agar supplies X and V factors for Haemophilus; MacConkey agar selects gram-negatives, with lactose fermenters turning pink; CNA inhibits gram-negatives for gram-positive recovery; thioglycollate broth supports anaerobes and reveals oxygen tolerance by growth depth.
Identification
Biochemical panels, oxidase, catalase, coagulase, and MALDI-TOF mass spectrometry. Coagulase-positive staphylococci are Staphylococcus aureus; oxidase-positive gram-negative rods include Pseudomonas; lactose-negative, H2S-positive colonies on MacConkey suggest Salmonella.
Susceptibility
The MIC is the lowest concentration inhibiting visible growth; disk diffusion (Kirby-Bauer) is interpreted by zone diameter against CLSI breakpoints, and broth microdilution yields the MIC directly. Breakpoints are revised as resistance emerges; ESBL, AmpC, MRSA (mecA), VRE (vanA/vanB), and carbapenemases (KPC, NDM, OXA-48) require specific reporting, often including intrinsic resistance to related classes.
Worked example
A CSF Gram stain shows gram-negative diplococci within neutrophils, and the isolate is oxidase positive. This is Neisseria meningitidis until proven otherwise; the result must be phoned to the clinician immediately, because turnaround here is a patient safety issue, not a documentation task.
Traps that cost marks
Reporting a probable contaminant (coagulase-negative staphylococcus in one of two blood culture sets) as a pathogen. Reading a disk zone across a swarming Proteus. Culturing a sputum specimen with many squamous epithelial cells, which reflects oral contamination rather than lower respiratory disease.