12. Effect of Physical Factors on Microorganism – Sterilisation & Sterilisation Methods
Explains sterilisation as the complete killing of all microorganisms including spores, then separates physical methods from chemical ones. Physical methods covered are moist heat in an autoclave with its standard temperature and pressure settings, dry heat in a hot air oven, UV and ionising radiation, and membrane and depth filtration. Chemical sterilants are then listed with concentrations and uses, including ethylene oxide, hydrogen peroxide and peracetic acid. The note closes by ranking microbes from most to least resistant to sterilisation, with prions at the top and mycoplasmas at the bottom.
Topic 12 of 113 in the MU Sofia syllabus for Microbiology (Dental Medicine, Year 2). The free AceMed note covers it in 2 sections, about 924 words (6 min of reading), with 13 flashcards.
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Effect of Physical Factors on Microorganism – Sterilisation & Sterilisation Methods
Effects of Physical Factors on Microorganisms: Sterilisation and Sterilisation Methods
Effects of Physical Factors on Microorganisms: Sterilisation and Sterilisation Methods
Sterilisation is defined as the killing of all microorganisms (MO’s), including spores, resulting in the absence of all microbes. It is the process of making something entirely free from bacteria or other living organisms such as viruses or spores. Sterilisation protocols are typically reserved for laboratory, medical, manufacturing, and food industry settings where complete absence of potentially infectious agents is necessary.
Sterilisation can be accomplished through either physical or chemical means.
Physical Methods of Sterilisation
The application of heat is a simple, cheap, and effective method of killing pathogens. Other physical methods include radiation and filtration.
Heat Sterilisation: The application of heat irreversibly denatures proteins (enzymes) of microorganisms.
Moist Heat Sterilisation (Autoclave):
Considered the most reliable method.
Uses steam under pressure as the sterilising agent. Basically a large pressure cooker.
Standard parameters: 121°C for 15-20 minutes at 105 Pa (which is 1 atmosphere total pressure or 202 kPa) or 134°C for three minutes at 303 kPa (two atmospheres total pressure). In practical operation, heating, equalising, and cooling times must be added.
The high heat energy content of steam is transferred to items upon condensation, and microbial proteins denature more readily in a moist environment.
Sufficient to kill spores of clostridia species, including C. botulinum spores, with an adequate margin of safety.
Used to sterilise surgical equipment, dressings, and heat-resistant laboratory ware.
Boiling fluids was identified by Lazzaro Spallanzani in the 18th century as a way to sterilise and kill microorganisms, although boiling medical instruments, needles, or syringes does not constitute sterilisation as many bacterial spores are not killed by this method.
Dry Heat Sterilisation (Hot Air Oven):
Uses hot air for sterilisation.
Stimulates oxidative destruction of cells.
Guideline values: 160°C for 120 minutes (2 hours) or 180°C for 30 minutes (1 hour). Objects must reach these temperatures for the prescribed period.
Used for glass, porcelain, and metal items.
Radiation Sterilisation:
Nonionising Radiation (UV light):
Uses UV rays (280–200 nm wavelength). Specifically 254 nm wavelength is mentioned as used.
Mechanism: Breaks DNA double strands or causes structural changes in DNA (thymine dimers), preventing replication. Bacteria can undergo photoreactivation (repair damage done by UV in light), so it is most effective in the dark.
Rapidly absorbed by materials.
Used only to reduce airborne pathogen counts (e.g. surgical theatres, filling equipment) and to sterilise work surfaces and tools in labs and hospitals.
Does not kill spores.
Ionising Radiation (X-rays, Gamma rays, Electron beams):
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