36 METHODS OF STERILISATION 



4. After an interval of twenty minutes in the case 

 of fluid media, or thirty minutes in the case of solid 

 media, take off the lid and remove the basket with its 

 contents. 



5. Now, but not before, extinguish the gas. 



NOTE. After removing tubes, flasks, etc., from the steam 

 steriliser, they should be at once separated freely in order to pre- 

 vent moisture condensing upon the cotton-wool plugs and soaking 

 through into the interior of the tubes. 



This treatment will destroy any vegetative forms of 

 bacteria ; during the hours of cooling any spores present 

 will germinate, and the young organisms will be de- 

 stroyed by repeating the process twenty-four hours 

 later; a third sterilisation after a similar interval 

 makes assurance doubly sure. 



The method of sterilising by exposure to streaming 

 steam at 100 C. for twenty minutes on each of three 

 consecutive days is termed discontinuous or inter- 

 mittent sterilisation. 



Exposure to steam at 100 C. for a period of one or 

 two hours, or continuous sterilisation, cannot always be 

 depended upon and is therefore not to be recommended. 



Superheated steam i. e., steam under pressure (see 

 Pressure-temperature table, Appendix, page 500) in 

 sealed vessels at a temperature of 115 C. will destroy 

 both the vegetative and the sporing forms of bacteria 

 within fifteen minutes ; if the pressure is increased, and 

 the temperature raised to 120 C., the same end is at- 

 tained in ten minutes. This method was formerly em- 

 ployed for the sterilisation of media (and indeed is so 

 used in some laboratories still), but most workers 

 now realise that media subjected to this high tem- 

 perature undergo hydrolytic changes which render 

 them unsuitable for the cultivation of the more deli- 

 cate micro-organisms. The use of superheated steam 

 should be restricted almost entirely to the disinfection 

 of such contaminated articles, old cultivations, etc., 



