106 BACTERIA IN MILK 



bacterial development sets in, the rapidity of which depends 

 upon the temperature at which the sample is stored. The 

 organisms that have gained admittance to the milk do not all 

 find that substance a suitable medium for reproduction, but 

 certain classes develop rapidly and ultimately one or more of 

 these classes predominates. The bacteria that reproduce most 

 rapidly may be roughly divided into three groups according to 

 their biochemical characteristics, viz., acid producers, pro- 

 teolytic, and inert organisms. Ayers and Johnson ^^ made a 

 fourth general division by separating the alkali producers, but 

 this group is usually included in the inert group. The classifi- 

 cation was based upon the behaviour of the organisms on litmus 

 lactose gelatine, the acid producers being those capable of 

 producing red colonies, the proteolytic being liquefiers, and the 

 balance, having no well-defined characteristics on this medium, 

 the inert group. The acid producers may be subdivided into 

 two further groups according to their ability to ferment lactose 

 with the production of gas. This separates the coliform organ- 

 isms, which produce hydrogen and carbon dioxide from lac- 

 tose in addition to lactic acid, and the ordinary lactic acid 

 organisms which do not give any gaseous products. 



Although different samples of milk will all show varying 

 rates of development of the various groups, a general dis- 

 cussion of this point will, perhaps, be facilitated by consider- 

 ation of a concrete example. Table XLIV shows the results 

 of a daily examination of a sample of milk kept comparatively 

 cool. 



All three groups, in this example, developed rapidly, the 

 greatest relative increase being shown by the coliform organ- 

 isms, until a maximum was reached at the end of five days. At 

 this stage the acidity was 44° and this amount was evidently 

 sufficient either alone or in conjunction with the other products 

 of metabolism, to restrain the rate of production. The coli- 

 form organisms were the first to be affected, although the other 

 acid producers and to an even smaller degree, the liquefiers, 

 were restrained. On the tenth day the liquefiers commenced 



