Microbiology and Infection Control
Microbiology and infection control questions test the difference between cleaning, disinfection and sterilisation, and whether you can apply standard precautions to a described situation. PGIMER frequently asks which method is appropriate for a given item.
Sterilisation parameters
- Autoclave: 121°C at 15 psi for 15–20 minutes for routine loads; 134°C for flash cycles. It kills spores. Monitor with a biological indicator (Geobacillus stearothermophilus), chemical tape and the pressure gauge.
- Hot air oven: 160°C for 2 hours, for glassware and powders.
- Ethylene oxide for heat-sensitive items; glutaraldehyde 2% for high-level disinfection of semi-critical items such as endoscopes (20 minutes).
- Boiling does not sterilise; alcohol does not kill spores.
Spaulding classification
- Critical items enter sterile tissue (surgical instruments, catheters): sterilise.
- Semi-critical items touch mucous membranes (laryngoscope, endoscope): high-level disinfection.
- Non-critical items touch intact skin (BP cuff, stethoscope): low-level disinfection.
Worked example
A flexible gastroscope has been used and needs reprocessing. It is heat sensitive and contacts mucous membrane, so it is semi-critical: clean, soak in glutaraldehyde 2% for 20 minutes, rinse with sterile water, dry and hang vertically. Reasoning: autoclaving would destroy the optics, and plain alcohol is not sporicidal enough for a semi-critical item.
Practices
- Hand hygiene five moments: before patient contact, before an aseptic task, after body fluid exposure, after patient contact, after touching surroundings. Alcohol rub 20–30 seconds; soap and water 40–60 seconds, mandatory when hands are visibly soiled.
- Biomedical waste rules 2016: yellow for anatomical and soiled waste, red for contaminated recyclable plastic, white for sharps, blue for glass and metallic implants.
Traps
Saying alcohol sterilises, treating boiling as sterilisation, and forgetting to clean an instrument before disinfecting it, since organic matter inactivates disinfectants.