112 OUTLINES OF BACTERIOLOGY 



of the acids which is effected by the ammonia, that is freely 

 developed during this kind of decomposition. After three or four 

 days some anaerobic bacteria begin to predominate, especially Bac. 

 perfringens, and Bac. bifermentans sporogenes. These use up both 

 the proteids and the sugars and after about eight or ten days, all 

 the sugars have disappeared. The removal of the sugars results in a 

 predominance of anaerobic bacteria, which decompose only the proteids, 

 such as Bac. putrificans, Bac. putidus gracilis and Diplococcus magnus 

 anaerobius. These anaerobes usually predominate until all the proteids 

 are used up. The greater part of meat-decomposition is therefore 

 performed by anaerobic bacteria. The initial predominance of the 

 aerobic bacteria results in the removal of oxygen from parts near 

 the surface, and owing to the close texture of the meat, there is 

 very little free oxygen in the deeper layers. 



(b) The Decomposition of Milk. As milk contains about four per 

 cent, of the sugar lactose, the course of development is essentially 

 different to that which takes place in meat. The first organisms 

 to predominate are usually Bac. subtilis and its aerobic allies 

 which decompose proteids, also organisms like Bac. coli communis 

 and Streptococcus acidi lactici that ferment both proteids and 

 lactose. In a very short time, however, the milk is completely 

 monopolised by the lactic-acid bacteria which are able to multiply 

 in this medium at an enormous rate. They transform so much 

 lactose into lactic acid that all bacterial development is temporarily 

 arrested. Before this happens, however, a portion of the proteids 

 contained in milk will have been decomposed by them. If now 

 no air be permitted to enter, or if its supply be inadequate, no 

 further changes take place. But if air be freely allowed to enter, 

 as for example, when milk is exposed in open pans, some of the 

 higher fungi, especially Oidium lactis appear in the milk after a few 

 days' exposure. These split up a portion of the remaining proteids, 

 and also decompose the lactic acid. The latter process diminishes 

 the acidity of the milk, and after a few more days the lactic-acid 

 bacteria are again able to multiply. The same process is now 

 repeated, so that some more of the proteids and some more of the 

 lactic acid are decomposed. This alternating preponderance of the 

 bacteria and the higher fungi, goes on until the proteids and the 

 sugar are almost completely used up. Sometimes there may be 

 at the same time a subsidiary fermentation, set up by the butyric- 

 acid bacteria, which change the lactose into butyric and propionic 

 acids. These substances, when present, impart a very disagreeable 



