Sterilisation

Sterilisation kills or removes all microbes, including spores. The method depends on what is being sterilised.

Heat is the most effective and quickest method for articles that can stand it. It kills by denaturing proteins.

• Boiling at 100 °C for 30 minutes kills most microbes, but not all spores.
• Autoclaving: steam under pressure. At 15 psi, water boils at 121 °C, hot enough to kill even spores. How long the load is held depends on its size. Autoclaves are vertical or horizontal, with or without a jacket.
• Pasteurisation kills disease-causing microbes in milk without fully sterilising it, so the milk keeps its taste: LTLT (low temperature, long time) at 63 °C for 30 minutes; HTST (high temperature, short time) at 72 °C for 15 seconds; UHT (ultra-high temperature) at 140-150 °C for 1-3 seconds.
• Dry heat: inoculating loops are flamed red hot; the mouths of tubes are passed through a flame; glassware is baked in a hot air oven.

Radiation sterilises without heat, so it is called 'cold sterilisation'.

• UV light (200-280 nm, most effective at 260 nm) damages DNA by forming thymine dimers. It does not penetrate well, so it is used for surfaces and work hoods.
• Electron beams sterilise syringes, gloves and some foods.
• Gamma rays penetrate deeply, so they sterilise packed plastics, antibiotics and hormones.

Filtration is used for heat-sensitive liquids such as serum, antibiotics and urea solutions, which heat would destroy. They are passed through membranes with pores of 0.2-0.45 µm, too small for microbes to pass. HEPA filters clean the air in laboratories.

Chemicals kill microbes on surfaces and skin: 70% ethyl alcohol, isopropyl alcohol, 40% formaldehyde, heavy metals and hydrogen peroxide.