Microbiology
Microbiology in this paper covers the nature of microorganisms, sterilisation and disinfection, the chain of infection and pathogens commonly seen in Tamil Nadu hospitals.
Core rules
- Prokaryotes (bacteria) lack a true nucleus; fungi, protozoa and helminths are eukaryotes; viruses are obligate intracellular parasites with no independent metabolism.
- Gram stain sequence: crystal violet → iodine → decolouriser → safranin. Gram-positive organisms retain the purple; Gram-negative appear pink.
- Autoclave: 121 °C at 15 psi for 15–20 minutes - moist heat that kills spores. Hot air oven: 160 °C for 2 hours for glassware and powders.
- Disinfection reduces organisms on inanimate objects; sterilisation destroys all organisms including spores. 70% alcohol is more effective than 100% because water helps penetration.
- Chain of infection: agent → reservoir → portal of exit → mode of transmission → portal of entry → susceptible host. Break any link to prevent infection.
- Mycobacterium tuberculosis is acid-fast and demonstrated by the Ziehl–Neelsen stain.
Worked example
A ward must reprocess a flexible endoscope and a set of steel artery forceps. The forceps go into the autoclave. The endoscope cannot, because 121 °C moist heat destroys its optics and polymers, so after thorough cleaning it receives high-level disinfection with glutaraldehyde. Reasoning: the choice is dictated by heat stability of the item, not by which method is intrinsically stronger.
Traps that cost marks
- Using sterilisation and disinfection as synonyms.
- Believing autoclave tape proves sterility - it only shows the pack was processed.
- Forgetting that antibiotics are ineffective against viruses.
- Missing that spores are the benchmark for true sterilisation.