Sterilisation by Heat and Autoclave
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Direct answer
Heat sterilisation kills microorganisms and spores, with moist heat in an autoclave working faster and more reliably than dry heat. A standard autoclave cycle runs saturated steam at 121 °C and 15 psi for 15 minutes, while a hot-air oven needs 160 °C for 2 hours. Tyndallisation uses intermittent steaming at 100 °C on successive days for heat-sensitive media, allowing spores to germinate and be killed.
What you must remember
- Moist heat advantage: steam transfers heat quickly and denatures proteins more effectively than dry air does.
- Autoclave standard: saturated steam at 121 °C and 15 psi for 15 minutes for standard loads.
- Air removal: trapped air lowers the steam temperature, so pre-vacuum cycles are used for porous loads.
- Dry heat: a hot-air oven at 160 °C for 2 hours, or 170 °C for 1 hour, for glassware, powders and oils.
- Tyndallisation: steaming at 100 °C for 20 to 30 minutes on three successive days, with incubation between sessions.
- Boiling: 100 °C kills vegetative bacteria but does not reliably kill spores, so it is disinfection, not sterilisation.
- Pasteurisation: 63 °C for 30 minutes or 72 °C for 15 seconds, which reduces pathogens in milk without sterilising it.
- Validation: chemical indicator tape, the Bowie-Dick test and Geobacillus stearothermophilus spores confirm that the cycle worked.
Comparing the heat methods
Heat methods differ in temperature, exposure time and whether spores are killed. Examiners often combine these figures with the question of whether a method counts as sterilisation or only disinfection. The table compares the methods most often asked in examinations.
| Method | Condition | Kills spores | Typical use |
|---|---|---|---|
| Autoclave | 121 °C, 15 psi, 15 min | Yes | Media, dressings, instruments |
| Hot-air oven | 160 °C for 2 h | Yes | Glassware, powders, oils |
| Tyndallisation | 100 °C, 20 to 30 min on 3 days | Yes | Heat-sensitive media |
| Boiling | 100 °C, 10 to 30 min | No | Disinfection of utensils |
| Pasteurisation | 63 °C for 30 min or 72 °C for 15 s | No | Milk |
Autoclaving depends on saturated steam, so air must be removed first, because a pocket of air lowers the temperature and leaves spores alive. Dry heat works more slowly by oxidation, which is why the oven needs longer exposure. Both methods must be followed by checks that confirm the load actually reached the required condition.
Where the exam tries to catch you
Heat sterilisation questions usually test the temperature, the time or the difference between sterilisation and disinfection.
- Trap: taking boiling for 10 minutes as sterilisation Correct: boiling kills vegetative cells but spores survive, so it is disinfection rather than sterilisation.
- Trap: using a hot-air oven for rubber or moist media Correct: dry heat damages rubber and cannot sterilise media, so moist heat in an autoclave is used.
- Trap: trusting the timer alone to confirm an autoclave cycle Correct: air pockets can prevent sterilisation even when the timer runs, so chemical indicators and the Bowie-Dick test are used.
Frequently asked questions
What autoclave conditions are standard for sterilising culture media?
The standard autoclave condition for sterilising culture media is saturated steam at 121 °C and 15 psi for 15 minutes. These conditions kill bacterial spores in a standard load, provided that air has been removed from the chamber first.
Why is tyndallisation used for certain heat-sensitive media?
Tyndallisation is used for media containing sugars, gelatin or egg, which can be damaged by a full autoclave cycle. Repeated steaming at 100 °C kills vegetative cells, and spores germinate between sessions and are killed on the following day.
What temperature and time does a hot-air oven need for dry heat sterilisation?
A hot-air oven needs 160 °C for 2 hours, or 170 °C for 1 hour, to sterilise by dry heat. Dry heat suits glassware, powders and oils, which would be damaged or wetted by steam.
What is the Bowie-Dick test used for in an autoclave?
The Bowie-Dick test checks that an autoclave removes air and that steam penetrates a porous load. A chemical indicator sheet in the pack changes colour evenly when steam has reached the centre of the load.
Why is boiling not considered a method of sterilisation?
Boiling at 100 °C kills vegetative bacteria but does not reliably kill bacterial spores, so it counts as disinfection rather than sterilisation. Spores of Bacillus and Clostridium species can survive boiling for long periods.