TYPES OF BACTERIA FOUND IN MILK. 145 



Varieties of Lactic AM Bacteria. A very large number of ap- 

 parently different kinds of acid-forming bacteria have been obtained 

 from milk. The different varieties all agree in producing lactic 

 acid, but differ in some other slight points, recognized by bacteri- 

 ologists. To what extent these many varieties should be combined 

 so as to make a small number of groups, and to what extent they 

 should be kept separate, is a matter over which there is as yet no 

 agreement. It is known that the same bacterium can show differ- 

 ences under different conditions. The power of a bacterium to 

 curdle milk may be increased by proper laboratory methods, 

 and when we find that, of these numerous described types, some 

 differ from others only in the rapidity with which they curdle milk, 

 we naturally infer that the different results are brought about by 

 the same bacterium growing under slightly different conditions. 

 Those who have given the most attention to the subject are convinced 

 that the lactic acid-forming bacteria that have been described must 

 be reduced to a few types, though no one yet ventures to say how 

 few. 



Among them are three well-marked types, quite radically distinct 

 from each other, and each playing an important part in the dairy. 

 Two of them are the dairyman's friends, while the 

 other is always his foe." 



I. Bacterium acidi lactici, Streptococcus lacticus. 

 These two names are applied to the same organism. 

 The first name was originally given to it when it was 

 described as a short rod (Fig. 30). Recently it has 

 been claimed that it is not a rod, but a coccus, and 

 with this conception the second name has been given as 

 the only correct one. Which of these two names is more correctly 

 applied has not yet been settled. But whatever its name and 

 microscopic appearance, it is a quite well-known organism, with a 

 distinctive action on milk. This type of lactic acid bacterium 

 grows better when not in free contact with the air. It grows better 

 under the surfaces of media than on the surface, failing to make 

 any visible growth on the surface of potato and scarcely any on 

 agar culture slants (see page 313). In milk, however, it grows 

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