Paper I — 2021 May (Supplementary) (2010 Scheme) — Q1
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Paper I
2021 May (Supplementary) (2010 Scheme) · 40 marks · 120 min
Question
Define sterilization. Classify methods of sterilization. Describe in detail moist methods
below 100˚C, above 100˚C. (1+2+3+4)
Q110 marksEssays
Answer
Definition: Sterilization is the complete destruction or removal of all forms of microbial life, including the most resistant bacterial spores, from an object or surface.
Classification of methods of sterilization:
Physical methods:
Sunlight
Heat — dry heat (flaming, incineration, hot air oven) and moist heat (below 100°C, at 100°C, above 100°C)
Filtration (for heat-labile liquids and air)
Radiation — non-ionizing (UV) and ionizing (gamma rays, electron beams)
Chemical methods — alcohols, halogens, aldehydes (formaldehyde, glutaraldehyde), gaseous sterilants (ethylene oxide), phenolics, and others (chosen chemical agent used at sporicidal concentration/exposure for true sterilization, as opposed to disinfection).
Mechanical methods — filtration (also classifiable under physical methods above).
Moist heat methods below 100°C:
Pasteurization — holder method (63°C/30 min) or flash/HTST method (72°C/15–20 sec); kills pathogens in milk without significantly altering nutritive quality; a disinfection method, not true sterilization (spores survive).
Vaccine bath — 60°C for 1 hour, used in preparing killed bacterial vaccines.
Moist heat methods at/above 100°C:
At 100°C:
Boiling — 100°C for 10–30 minutes; kills vegetative organisms, not all spores.
Tyndallisation (intermittent sterilization) — steaming at 100°C for 20 minutes on three successive days with intervening incubation, exploiting spore germination between cycles; used for heat-labile fluids/media.
Steaming (Koch’s/Arnold’s steamer) — free-flowing steam at 100°C.
Above 100°C (under pressure):
Autoclaving — the standard, most reliable sterilization method; steam under pressure (15 lbs/sq inch → 121°C for 15–20 minutes) kills even bacterial spores by protein denaturation/coagulation; used for surgical instruments, culture media, dressings, and rubber goods.
Moist heat is generally more efficient than dry heat at equivalent temperatures because moisture facilitates protein denaturation at lower temperatures and in shorter exposure times.
