28 METHODS OF CULTIVATION OF BACTERIA 



some bad conductor of heat, such as felt or asbestos. A 

 perforated tin diaphragm is fitted in the interior at a little 

 distance above the bottom, and there is a tap at the bottom 

 by which water may be supplied or withdrawn. If water 

 to the depth of 3 inches be placed 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 standing 

 in its neck. The funnel may be supported by passing its tube 

 through a second perforated diaphragm placed in the upper part 

 of the steam chamber. 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 in the case 

 of media containing gelatin such an exposure is not practic- 

 able, as with long boiling, gelatin tends to lose its 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 nori-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 condition. 



Steam at 100 C. is therefore available for the sterilisation of 

 all ordinary media. In using the Koch's steriliser, especially 

 when a large bulk of medium is to be sterilised, it is best to 

 put the media in while the apparatus is cold, in order to make 

 certain that the whole of the food mass reaches the temperature 

 of 100 C. The period of exposure is reckoned from the time 

 boiling commences in the water in the steriliser. At any rate 

 allowance must always be made for the time required to raise 

 the temperature of the medium to that of the steam surrounding it. 



If we wish to use such a substance as blood serum as a 

 medium, the albumin would be coagulated by a temperature of 

 100 C. Therefore other means have to be adopted in this case. 



