vi] THE BACTERIA OF MILK 83 



bacteria causing acidity in milk may be included 

 under this class, since they are the necessary fore- 

 runners of other bacteria which only thrive in 

 strongly acid media, and which are therefore most 

 frequently found in sour milk. As an example of 

 this class may be mentioned the butyric bacilli, 

 which impart to milk a strongly bitter flavour, but 

 which are most familiar through their action in 

 rancid butter. 



IV. Bacteria ivhose Action is of a Directly Injurious 

 Nature. Fourthly, there are bacteria which exert a 

 directly injurious action on milk, and which are 

 therefore of greatest interest in this connection. 

 This last class of bacteria may be divided into 

 different sub- classes. Thus we have one kind which 

 produces acidity, and of which the lactic bacteria 

 and certain colour-producing bacteria, such as the 

 bacillus prodigiosus, may be cited as examples. To 

 this sub-class also belong bacteria producing butyric 

 fermentation, which develop in the absence of air, 

 and which produce a slight coagulation of the 

 caseous matter of the milk, which becomes subse- 

 quently dissolved ; as also bacteria producing volatile 

 acids, under the action of which milk generally 

 assumes a grayish colour and becomes rather viscous, 

 while a lively generation of gas often takes place, 

 consisting of carbonic acid, sulphuretted hydrogen, 

 and, it may be, small quantities of alcohol and acetic 



CFTHE 



UNIVERSITY, 



