THE METHODS OF STERILISATION. 35 



cultures is absolutely essential. If two or more different 

 organisms be present together, we cannot say that any one 

 of them is the cause of the disease in question. 



To obtain pure cultures, then, is the first requisite of 

 bacteriological research. Now, as bacteria are practically 

 omnipresent, vvve must first of all have means of destroying 

 all extraneous organisms which may be present in the food 

 media to be subsequently used for growing the bacteria we 

 wish to study, in the vessels in which the food media are 

 contained, and on all instruments which are to come in 

 contact with our cultures. The technique of this destructive 

 process is called sterilisation. We must therefore study the 

 methods of sterilisation. The growth of bacteria in other 

 than their natural surroundings involves further the prepara- 

 tion of sterile artificial food media, and when we have such 

 media prepared we have still to look at the technique of the 

 separation of micro-organisms from mixtures of these, and 

 the maintaining of pure cultures when these have been obtained. 

 We shall here find that different methods are necessary 

 according as we are dealing with aerobes or ancerobes. 

 Each of these methods will be considered in turn. 



THE METHODS OF STERILISATION. 



To exclude extraneous organisms, all food materials, 

 glass vessels containing them, wires used in transferring 

 bacteria from one culture medium to another, instruments 

 used in making autopsies, etc., must be sterilised. These, 

 objects being so different, various methods are necessary. 

 The foods comprise meat infusions, jellies, potatoes, etc., 

 and a method suitable for their sterilisation evidently may 

 not be suitable for the sterilisation of, say, a glass flask. 

 Bacteria may be killed by various methods. Many chemicals 

 will kill them, but the difficulty of subsequently removing 

 such chemicals, so that they may not interfere with the 

 growth of the microbes we wish to cultivate, makes their 

 use inapplicable. We therefore in practice take advantage 

 of the principle that all bacteria are destroyed by heat. 



