Microbiology and Immunology: organisms, mechanisms and hypersensitivity
Scope
Expect bacterial structure and staining, culture and sterilisation, mechanisms of antibiotics, viral replication, mycology, parasitology, and the innate and adaptive immune responses including hypersensitivity and vaccines.
Core rules
- Gram stain separates organisms with a thick peptidoglycan layer (Gram positive, purple) from those with a thin layer and an outer membrane (Gram negative, pink).
- Sterilisation. Autoclaving runs at 121 degrees Celsius and 15 psi for 15 minutes; its biological indicator is Geobacillus stearothermophilus spores.
- Hypersensitivity. Type I is IgE and mast cells; type II is antibody against cell-bound antigen; type III is immune complex deposition; type IV is T-cell mediated and delayed.
- Antibiotic targets: cell wall (beta-lactams, vancomycin), protein synthesis (aminoglycosides, macrolides), nucleic acid (fluoroquinolones), folate pathway (trimethoprim–sulfamethoxazole).
Worked example
A child develops fever, urticaria, joint pains and proteinuria 10 days after a course of penicillin. There is no immediate anaphylaxis. Reasoning: the delay, rash and glomerular involvement point to circulating antigen–antibody complexes depositing in small vessels, which is a type III reaction. Type I would have occurred within minutes, type II would have caused haemolysis, and type IV would have produced a contact dermatitis.
Traps that cost marks
- Labelling any rash after a drug as allergy without identifying the mechanism.
- Confusing sterilisation with disinfection; only sterilisation kills spores.
- Forgetting that bacterial meningitis shows neutrophil predominance, low glucose and high protein, while viral meningitis shows lymphocytes and normal glucose.
- Assuming all Gram-positive cocci in clusters are streptococci - clusters suggest staphylococci.