- It is defined as the process by which an article, surface or medium is freed of all living microorganisms either in the vegitative or spore state.
1. Physical agents
- It is the most reliable method of sterilisation.
Mechanism of action-
- Killing effect of dry heat is due to protein denaturation, damaging by oxidising molecules, destroying the cell constituents and the toxic effect of elevated levels of electrolytes.
- The lethal effect of moist heat is due to denaturation and coagulation of proteins.
- Thermal death time is inversely proportional to the temperature of exposure
- The sterilisation time is related to the number of organisms in the suspension, presence or absence of spores and the strain and the characteristics of the organism.
- It does not include time taken to reach the specified temperature.
- The presence of disinfectants and high acidic or alkaline pH hastens bacterial killing.
- Inoculating loop or wire, the tip of forceps and searing spatulas can be sterilised by holding them over a Bunsen flame till they become red hot.
2. Incineration –
- It is the excellent method for terminal sterilisation for destroying biomedical waste.
- Plastics such as PVC can be dealt with it.
3. Hot air oven-
- Sterilisation is achieved by conduction.
- A holding period of 160℃ for two hours is necessary to sterilise glassware, forceps, scissors, scalpels, all glass syringes, swabs etc.
- At 180℃ cotton plugs may get charred.
- At 150℃ sharp instruments, such as those used in ophthalmic surgery should be sterilised.
Temperature below 100℃-
1. Pasteurisation of milk-
- The milk is heated at either 63℃ for 30 minutes (the holder method) or 72℃ for 15-20 seconds (the flash process), followed by cooling quickly to 13℃ or lower.
- Media such as Lowenstien-jensen and Loefller’s serum are rendered sterile by heating at 80-85℃ for half an hour on three successive days in an inspissator.
- Bacterial vaccines are heat inactivated in the special vaccine baths at 60℃ for one hour.
- Serum or body fluids containing the coagulation proteins can be sterilised by heating for one hour at 56℃ in a water bath on several successive days.
Temperature at 100℃-
- Vegetative bacteria are killed almost immediately at this temperature.
- Boiling is not recommended for sterilisation of instruments used for surgical procedures.
- Hard water should not be used for boiling.
- Addition of 2% sodium bicarbonate in the water make it soft and make it suitable for sterilisation.
2. Steam at atmospheric pressure-
- It is used for culture media.
- A Koch and Arnold steamer is usually used.
Tyndallisation or intermittent sterilisation-
- It is used for media containing sugars or gelatin.
- An exposure to 100℃ for 20 minutes on three successive days is used.
- The principal is that the first exposure kill all the spores, since they are in favourable medium, will germinate and killed on the subsequent exposure.
- However this method may fail to kill spore of certain anaerobes and thermophiles.
Steam under pressure-
- The equipment used is autoclave.
- Sterilisation by steam under pressure is carried out at the temperatures between 108℃ and 147℃.
- Aqueous solutions are sterilised between 108℃ and 126℃.
- Autoclaves ( steam seteriliser ) are used in healthcare setup: laboratory autoclaves, hospital dressing steriliser, instrument steriliser, rapid cooling steriliser.
- Two types of autoclaves are available: gravity displacement type and high vacuum sterilisers.
- It helps in removing bacteria from heat labile liquids such as sera and solutions of sugars and antibiotics.
- Viruses can pass through ordinary filters.
- Bacterial toxins can be obtained by passing cultures through filters.
- Asbestos filters are no longer used due to its carcinogenic property.
Types of filters-
1. Candle filters – have been widely used for purification of water for industrial and drinking purposes.
2. Sintered glass filters
3. Membrane filters- are routinely used in water purification and analysis, sterilisation and sterility testing, and for the preparation of solutions of parenteral use.
- Two types of radiation are used for sterilisation: non ionising and ionising.
- Non ionising radiation- infrared and ultraviolet rays.
- Ionising radiation- gamma rays and high energy electrons.