Microbiology
Specimen selection and culture interpretation
Microbiology results depend on site, timing, and contamination control. Blood cultures should be drawn before antibiotics when possible, with adequate volume per bottle; volume is a major sensitivity factor. CSF requires prompt transport and processing because cells and fastidious organisms deteriorate. Sputum acceptability is judged by squamous epithelial cells and neutrophils; a specimen with many squamous cells represents oral contamination. Urine culture interpretation uses colony count plus symptoms; mixed flora often means contamination. Wound and abscess specimens should be collected from the active margin, not superficial exudate, and anaerobic specimens need appropriate transport. Gram stain gives early information: shape, arrangement, and Gram reaction. Culture media are selected for the site (blood agar, chocolate agar, MacConkey, anaerobic media). Identification uses biochemical, antigenic, and molecular methods. Susceptibility testing reports MIC or disk-diffusion interpretation; breakpoints are organism- and site-specific. MRSA, ESBL, and carbapenemase questions test infection-control and reporting implications, not just drug names.
Traps and worked example
Traps: calling a positive urine culture infection without symptoms, ignoring sputum quality, and submitting a swab instead of tissue for anaerobic culture. Worked example: a clean-catch urine with 10^4 CFU/mL of a single uropathogen in a symptomatic patient can be significant, while 10^5 CFU/mL of mixed flora in an asymptomatic patient usually suggests contamination. Rule of thumb: interpret colony count with the patient and collection method; never report a culture in isolation from specimen quality.