Microbiology
What to master
Build a decision tree from specimen to organism to susceptibility. Know Gram stain reaction and morphology, then media: sheep blood agar, chocolate, MacConkey (lactose fermenters pink), Columbia colistin-nalidixic acid, mannitol salt, bile esculin, XLD and Hektoen, thioglycollate, and anaerobic media. Spot tests: catalase, oxidase, coagulase, PYR, optochin, bacitracin, bile solubility, hippurate. Streptococcus grouping by Lancefield antigen; Enterococcus by bile esculin plus 6.5 % salt. Enterobacterales are oxidase-negative glucose fermenters identified with TSI, indole, citrate, urease, and motility. Nonfermenters such as Pseudomonas are oxidase-positive with grape-like odor and blue-green pigment. Know blood culture volume and set requirements, urine colony-count thresholds, and the C. difficile testing algorithm (GDH plus toxin with NAAT resolution).
Susceptibility: MIC versus disk diffusion, intrinsic resistance, and marker tests for ESBL, carbapenemase, MRSA (mecA/PBP2a), and VRE (vanA/vanB).
Traps
Distinguish contamination from infection (coagulase-negative staphylococci in one of two bottles), recognize normal respiratory and genital flora, and account for cultures drawn after antibiotics.
Worked example
A beta-hemolytic colony that is bacitracin susceptible and PYR positive is Streptococcus pyogenes (group A). A beta-hemolytic colony that is bacitracin resistant and hippurate positive is Streptococcus agalactiae (group B).