METHODS OF CULTIVATION OF BACTERIA. 



is a tap at the bottom by which water may be supplied or 

 withdrawn. If water to the depth of 3 inches be placed 

 n in the interior and heat applied, it will 



quickly boil, and the steam streaming 

 up will surround any flask or other object 

 standing on the diaphragm. Here no 

 evaporation takes place from any medium 

 as it is surrounded during sterilisation 

 by an atmosphere saturated with water 

 vapour. It is convenient to have the 

 cylinder tall enough to hold a litre flask 

 with a funnel 7 inches in diameter stand- 

 ing in its neck. With such a " Koch " 

 in the laboratory a hot-water filter is not 

 needed. As has been said, one and a 

 half hour's steaming will sterilise any 

 medium, but as some of our most im- 

 portant media contain gelatine, such 

 an exposure is not practicable, as with 

 FIG. 3 . -Koch's steam ^ng boiling, gelatine tends to lose its 

 steriliser. physical property of solidification. The 



method adopted in this case is to steam 

 for a quarter of an hour on each of three succeeding days. This 

 is a modification of what is known as " Tyndall's intermittent 

 sterilisation." The fundamental principle of this method 

 is that all bacteria in a non-spored form are killed by 

 the temperature of boiling water, while if in a spored form 

 they may not be thus killed. Thus by the sterilisation 

 on the first day all the non-spored forms are destroyed 

 the spores remaining alive. During the twenty-four hours 

 which intervene before the next heating, these spores, being 

 in a favourable medium, are likely to assume the non- 

 spored form. The next heating kills these. In case any 

 may still not have changed their spored form, the process is 

 repeated on a third day. Experience shows that usually 

 the medium can now be kept indefinitely in a sterile con- 

 dition. Steam at 100 C. is therefore available for the 

 sterilisation of all ordinary media. In using the Koch's 



