TYPES OF BACTERIA FOUND IN MILK 157 
produce the storm hasten the growth of bacteria. A similar warm 
spell will sour the milk just as quickly, even though no thunder 
storm appears. Whether this is the whole explanation may be 
doubtful, but it is clearly demonstrated that the thunder and 
lightning have nothing to do directly with the phenomenon. The 
souring of milk is always produced by bacteria. 
Varieties of Lactic Acid Bacteria—A very large number of ap- 
parently different kinds of acid-forming bacteria have been ob- 
tained from milk. The different varieties all agree in producing 
lactic acid, but differ in some other slight points, recognized by 
bacteriologists. 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 yetno agreement. Itis known that the same bacterium can 
show differences under different conditions. The power of a bac- 
terium 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 sub- 
ject 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 lactis acidt, 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. 
29). Recently it has been claimed that it is notarod, butacoccus, 
and with this conception the second name has been given as the 
only correct one. Which of these two names is more correctly 
