Microbiology
Microbiology tests organism identification, culture media, antibiotic mechanisms, resistance patterns and sterilisation practice.
Identification
- Gram-positive cocci in clusters: staphylococci; Staphylococcus aureus is coagulase positive.
- Gram-positive cocci in chains: streptococci.
- Gram-negative bacilli: enterobacterales and pseudomonas.
- Media: blood agar for most pathogens, MacConkey for lactose fermenters (pink colonies), chocolate agar for Haemophilus, Löwenstein-Jensen for mycobacteria.
Antibiotic mechanisms
- Cell wall: beta-lactams, vancomycin.
- Protein synthesis: aminoglycosides and tetracyclines (30S), macrolides and clindamycin (50S).
- Folate pathway: sulfonamides and trimethoprim.
- DNA gyrase: fluoroquinolones.
- RNA polymerase: rifampicin.
Resistance
- MRSA: mecA gene, altered penicillin-binding protein; treat with vancomycin.
- ESBL producers: resistant to cephalosporins; use carbapenems.
- VRE: altered cell wall target.
Sterilisation
Autoclaving at 121°C for 15 minutes kills spores; disinfection does not. Needlestick exposure carries the highest transmission risk for hepatitis B, and post-exposure prophylaxis exists.
Worked example
A wound swab grows gram-positive cocci in clusters that are catalase and coagulase positive. This is Staphylococcus aureus; if it is methicillin resistant, vancomycin is the appropriate choice, and the drainable collection should be drained.
Traps
Confusing sterilisation with disinfection, choosing cephalosporins for ESBL organisms, and forgetting that antibiotics alone do not treat an abscess.