MILK— CHEESE INFUSION. 109 



quite sterile, and may be used as a culture medium for a great 

 number of organised species (torulae, bacilli, micrococci). As obtained 

 from ordinary sources, milk must be thoroughly sterilised before use. 

 Its sterilisation is more than usually difficult, from its physical 

 character as a bad conductor of heat, and from the fact that it is 

 early contaminated with germinal matter in the usual methods of its 

 storage and transmission. This germinal matter consists, amongst 

 other elements, of the dried spores of Bacterium lactis (Lister) and 

 Bacillus butyris (Pasteur, Cohn), which, when brought into contact 

 with fresh milk under suitable conditions of temperature, &c, rapidly 

 take on vegetative activity, and render it difficult of subsequent 

 sterilisation. The method of destroying this activity, and rendering 

 the milk sterile, is by steaming that liquid repeatedly for thirty 

 minutes at a time, at intervals of twenty-four hours, for five or six 

 days. If it be only twice steamed, as prescribed for other animal 

 liquids, B. lactis is usually excluded, but after an interval it is often 

 found that the cream layer has become rancid, owing to the develop- 

 ment of B. butyris, although the milk beneath that layer may remain 

 quite pure. When thoroughly sterilised, milk should be incubated 

 at 32° C. under observation for a week, and if it remain unaltered, it 

 may then be used as a culture medium. 



Blood Serum. 



55. Limpid serum is obtained from the various sources (blood 

 from slaughter-houses, hydroceles, pleuritic and peritonitic effusions, 

 blisters, &c.) already mentioned when treating of the solid media 

 (see § 48, p. 102). It is sterilised by the slow process as previously 

 described, and it can be used as a culture medium in the various 

 ways enumerated under " animal infusions." 



Cheese Infusion. 



56. Infusions which contain cheese as one of their constituents 

 must be sterilised with the greatest circumspection, and afterwards sub- 

 jected to the test of prolonged incubation. This is rendered necessary 

 by the great resistance which cheese offers to the sterilising process. 

 The quality of impermeability by heat, x which it possesses in common 

 with milk, accounts for this difficulty in effecting its sterilisation. 



1 Tyndall, Proc. Roy. Soc, 1877, vol. 26, p. 228. 



