BACTERIA AND THEIR REEATION TO DAIRYING. 15 



Sterilization of Culture Media. When these nutritive 

 media are used for the culture of bacteria they must be 

 sterile, i.e., free from all living organisms, as the culture 

 would otherwise be impure from bacteria found in the sub- 

 stratum before the organism to be studied was seeded or 

 inoculated in it. Sterilization is usually effected by heat- 

 ing to such a temperature that both bacteria and their 

 spores are killed. As we saw in the preceding, the vitality 

 of different species of bacteria differs greatly : while some 

 bacteria are killed at rather low temperatures, others re- 

 quire a temperature considerably above the boiling point. 

 To be absolutely certain that all bacteria, and above all 

 their spores, are destroyed, we should heat intensively for 

 every sterilization. In some cases this is impossible, 

 as many substrata cannot be heated very high without 

 being changed in one way or another. In case of such 

 nutritive substrata we must therefore make use of other 

 methods of procedure. A lower heat acting for a longer 

 time or repeatedly may, e.g., be applied. This last method 

 (intermittent sterilization) has to be used, among other 

 cases, in the sterilization of milk and its products. 



Intermittent Sterilization. Having killed the vegeta- 

 tive forms of the bacteria through a careful heating, the 

 substratum is allowed to stand at ordinary temperature for 

 some time, so that the spores, which of course retained their 

 vitality, may develop into ordinary* bacteria; and these are 

 then in their turn killed by a second heating. As all 

 spores may not then have reached the vegetative stage, 

 or as some new spores may have been formed during the 

 interval, it is safest to repeat the operation one or more 

 times, according to necessities in each case. To be entirely 

 safe, the milk is never used until its sterility has been 



