Microbiology: structure, stain and the toxin question
Microbiology questions here are constructed from a short clinical stem plus one discriminator: the Gram reaction, the toxin, the culture medium or the route of transmission. Learn organisms as profiles rather than as isolated facts, and learn sterilisation as a hierarchy of physical and chemical methods.
Core rules
- Gram-positive cell walls are thick peptidoglycan; Gram-negative walls have an outer membrane whose lipid A is the endotoxin responsible for septic shock.
- Exotoxins are proteins, heat-labile, often plasmid-encoded and can be converted to toxoids; endotoxin is heat-stable lipopolysaccharide and cannot.
- Catalase and coagulase separate staphylococci; bacitracin and bile solubility separate streptococcal groups.
- Sterilisation kills all organisms including spores (autoclave 121 degrees Celsius at 15 psi for 15 minutes); disinfection reduces pathogens on inanimate surfaces and does not guarantee spore kill.
- Resistance mechanisms worth naming: beta-lactamase, altered penicillin-binding proteins (MRSA), efflux pumps, and porin loss.
Worked example
A 24-year-old has fever, neck stiffness and photophobia. Cerebrospinal fluid shows 4000 white cells per microlitre with 90% neutrophils, protein 2.5 g/L, glucose 1.2 mmol/L against a blood glucose of 5.5 mmol/L. The cell differential and the low CSF-to-blood glucose ratio favour bacterial meningitis, so the decision is empirical intravenous antibiotics after blood cultures, not a wait for viral serology. A viral picture would show a lymphocytic pleocytosis, normal or mildly raised protein and normal glucose.
Traps that cost marks
- A raised CSF protein alone does not distinguish bacterial from viral disease; the ratio of CSF to blood glucose does more work.
- Antibiotics do not treat viral infection, and prophylaxis is not the same as treatment.
- Hand hygiene remains the single most effective interruption of hospital transmission.
- Culture media pairings are asked directly: Lowenstein-Jensen for mycobacteria, chocolate agar for Haemophilus.