Microbiology
Microbiology - what the exam tests
Microbiology (Domain 3) examines bacteria, viruses, fungi and parasites, the chain of infection, sterilisation and disinfection, hospital-acquired infection, immunity and the rational use of antibiotics.
Core rules
- Chain of infection: infectious agent → reservoir → portal of exit → mode of transmission → portal of entry → susceptible host. Break one link to prevent infection.
- Sterilisation destroys all organisms including spores; disinfection reduces pathogens on inanimate objects but may not kill spores.
- Autoclave: 121 °C at 15 psi for 15–20 minutes (or 134 °C for 3–4 minutes). Boiling at 100 °C kills most vegetative bacteria but not spores.
- Hand hygiene is the single most effective measure; alcohol rub for visibly clean hands, soap and water when soiled and for Clostridioides difficile.
- Gram stain: crystal violet → iodine → decolouriser → safranin; Gram-positive organisms stay purple, Gram-negative appear pink.
Worked example
A dressing trolley has been wiped with 70 % alcohol. Is it sterile? No. Alcohol is a disinfectant: it kills many vegetative bacteria and some viruses but does not reliably destroy bacterial spores such as Clostridium tetani, nor all viruses. For a sterile procedure the instruments must be autoclaved; the trolley is only a clean, disinfected work surface, and the sterile field is created by the sterile drape.
Traps that cost marks
- Claiming antibiotics act on viruses or the common cold.
- Saying boiling or sunlight 'sterilises'.
- Forgetting that an autoclave must be loaded loosely so steam penetrates, and that chemical indicators are not biological spore tests.
- Ignoring the required contact time and dilution of a disinfectant.