BACTEEIOLOGY OF MILK 449 



bacteria. In some cases, however, particularly when there 

 is unusually heavy inoculation of other forms, the latter 

 may develop and the milk will not show normal souring. 

 Milk, for example, which has been heated nearly to the 

 boiling temperature, will contain no lactic acid bacteria. 

 The spores of many of the putrefactive and decay-pro- 

 ducing forms, however, will remain. These will develop 

 when the milk is maintained at room temperature and lead 

 to various changes such as sweet curdling. Many of the 

 organisms belonging to the group of Bacilliis subtilis bring 

 about this change. The sweet curdling is due to the action 

 of a rennetlike enzyme produced by the bacteria. In most 

 cases the microorganisms proceed to decompose the curd, 

 transforming it into amino acids and ammonia. 



Under the discussion of capsule formation by micro- 

 organisms, it has already been noted that there are many 

 species which are capable of producing sliminess or ropiness 

 in milk. Some of these species are aerobic and produce the 

 sliminess near the surface of the medium in contact with the 

 air. Other forms apparently are facultative and produce 

 sliminess throughout the medium. 



Pigment-producing bacteria sometimes cause trouble in 

 milk. Species are known which form blue islands on the 

 surface of the milk, others change the milk itself to red, 

 yellow or even brown or black. Other species have been 

 described which produce undesirable flavors, in some cases 

 developing bitter milk and soapy milk. 



BACTERIAL INFECTION OF MILK 



Bacteria gain access to milk in a variety of ways. It is 

 of considerable practical significance, therefore, to know of 

 the sources of most importance. It is evident that some 

 sources of bacterial contamination are important because of 

 the numbers of bacteria which they will contribute, others 

 are important rather because of the kinds. 



