Microbiology · Short notes

Sterilization: Short Notes & Cheat Sheet

Sterilisation versus disinfection, heat, filtration, radiation and gas methods, biological indicators, D and Z values, disinfectants and the Spaulding classification.

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Short notes

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Resistance to killing, from hardest to easiest

Choose the process for the most resistant organism likely to be present. Only sterilisation reliably kills spores.

Definitions

Sterilisation kills or removes all microorganisms, including bacterial spores.
Disinfection reduces pathogens on inanimate objects but may not kill spores. Antisepsis does the same on living tissue.
D value: time at a given temperature to kill 90% of the organisms (one log reduction). Z value: the temperature change that alters the D value tenfold.
F₀ is the lethality of a heat process expressed as equivalent minutes at 121 °C. The target sterility assurance level is 10⁻⁶.

Methods

Moist heat (autoclave): 121 °C for 15 minutes at about 15 psi, or 134 °C for about 3 minutes. It is the most reliable method, for aqueous solutions, dressings and instruments.
Dry heat (hot-air oven): 160 °C for 2 hours (or 170 °C for 1 hour, 180 °C for 30 minutes). Used for glassware, oils and powders that steam cannot penetrate.
Filtration through 0.22 µm membranes suits heat-sensitive solutions, but does not reliably remove viruses or mycoplasma. HEPA filters clean air.
Gamma radiation (cobalt-60) is 'cold sterilisation' for single-use plastics and devices. UV light only disinfects surfaces and air, because it penetrates poorly.
Ethylene oxide gas sterilises heat-sensitive devices and plastics, but it is toxic, carcinogenic and flammable, and items need aeration afterwards.
Pasteurisation (63 °C for 30 minutes or 72 °C for 15 seconds) and tyndallisation are not full sterilisation methods.

Disinfectants

Alcohol works best at 60–90% (70% is typical), because water is needed to denature proteins. It does not kill spores.
Glutaraldehyde 2% is a high-level disinfectant for endoscopes; with long contact it is sporicidal.
Chlorhexidine is a skin antiseptic and mouthwash. Quaternary ammonium compounds (benzalkonium) are inactivated by soap.
Hypochlorite (bleach) is cheap, broad-spectrum and used for spills and water treatment. Phenol is the reference standard in the Rideal–Walker and Chick–Martin tests.
Spaulding classification: critical items (enter sterile tissue) must be sterilised; semi-critical items (touch mucous membranes) need high-level disinfection; non-critical items (touch intact skin) need low-level disinfection.

Cheat sheet

High-yield facts in one place. Cover the right-hand column and test yourself.

Method → conditions → indicator

Method	Conditions · biological indicator
Autoclave	121 °C, 15 min, 15 psi · Geobacillus stearothermophilus
Hot-air oven	160 °C, 2 h · Bacillus atrophaeus
Ethylene oxide	Gas, 30–60 °C · Bacillus atrophaeus
Gamma radiation	Cobalt-60 · Bacillus pumilus
Filtration	0.22 µm membrane · Brevundimonas diminuta

Spaulding classification

Item	Process
Critical (surgical instruments)	Sterilisation
Semi-critical (endoscopes)	High-level disinfection
Non-critical (BP cuff)	Low-level disinfection

Memory aids

121 for 15 at 15: Autoclave at 121 °C for 15 minutes at 15 psi.

Exam traps

70% alcohol kills better than absolute alcohol.
Dry heat needs a higher temperature and longer time than moist heat.
UV radiation disinfects surfaces; it is not a reliable method for sterilising products.
A 0.22 µm filter does not reliably remove viruses.

Revised? Now test yourself.

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