The killing action of heat on the organisms can be done by using increase in the steam in a closed system. The water molecules become aggregated resulting in increase in their penetration. The water boils at 100 C and the steam accumulates in a closed container resulting in increase in pressure.

(I) The autoclave is usually of pressure cooker type made up of gun metal sheets which is supported in a iron case.

(ii) It is closed by swing door which is fastened by radical bolts tightly. 

(iii) In microbiology laboratories, system jacketed horizontal type autoclave is necessary.

(iv) The steam passes from below at the base. The side walls are heated by the steam jacket. It has a provision to record the pressure.

(v) There is a possibility to regulate the pressure using pressure meter. It consists of safety valve that guards against the accidents. It is based on moist heat that used in sterilisation.

(vi) The autoclave is usually operated at 15 lb./inch2  steam pressure for 30 min. This

temperature for 30 minutes is enough to kill all the spores and cells of microorganisms.


The level of water should be checked before operating. The air should be completely evacuated and the steam must have access to the materials to be sterilised. The cotton or glass beads must be sterilised in a glass container closed with foil. The heat sensitiser substances should not be sterilised by autoclaving. Now-a-days, checking of complete autoclaving is possible by using strips contains red solution which turns green, at 115 C , if kept for 25 min. The use of bacilli spores, after autoclaving is considered perfect if no growth is obtained on thioglycollate medium or cooked meat medium.

Gaurav Singh

Editor in Chief Medical Microbiology & RDT Labs - RDT Labs Magazine

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