BACTERIA IN MILK 185 



Souring of Milk. The souring of milk is due to a variety 

 of bacteria, chief among which is Bacterium lactis aerogenes, 

 related to the Bacterium bulgaricum described above. This 

 germ is ubiquitous. It is not pathogenic. It produces a 

 fermentation of the sugar of milk, lactose, into lactic acid. 

 Moulds may help this and oftentimes lactic acid and ethyl 

 alcohol may be formed side by side. The latter predominates 

 in the carbonated milks like koumyss. Other bacteria cause 

 clot, or precipitation of the casein, the forerunner of cheese. 

 Streptococci from the udder or manure may also help in 

 souring. 



To make buttermilk in the home is a simple matter. A 

 quantity of whole or skim milk is boiled and cooled. A 

 tablet containing the lactic acid bacilli, a small quantity 

 of pure culture of the organism, or a "starter" from a pre- 

 vious making is then added to this cooled milk and set aside 

 in a warm room (about 75 F.) overnight. The result is a 

 rather agreeable sour milk. Pharmaceutical chemists and 

 laboratories are now supplying tablets and cultures for this 

 purpose. (See page 166.) 



Diseases Caused by Polluted Milk. Many diseases are 

 believed to be due to bad or polluted milk. If milk merely 

 carry the germs this is easily understood, but, as is the case 

 in the diarrheas of infants, the trouble may lie not with the 

 bacteria introduced with the milk, but with the disturbance 

 of digestion caused by the abnormal chemical conditions 

 brought about by souring. These strange chemical sub- 

 stances so prevent normal digestion that really pathogenic 

 bacteria, the dysentery bacillus group, for example, are able 

 to exert their noxious effects. Streptococci commonly present 

 in the teats, identical with the Streptococcus pyogenes, are 

 said by some to take advantage of this disturbed digestion. 

 The examination for streptococci consists in simple staining 

 and finding of them lying in or about pus cells. Health 

 authorities have rules covering this method of examination 

 and the interpretation of results. 



