TYPES OF BACTERIA FOUND IN MILK. 145 
Varieties of Lactic Acid 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 undef 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 acidi I act id 
been claimed that it is not a rod, but a coccus, and LS!)"* 
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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