146 BACTERIA IN MILK. 
with great rapidity, soon turning it acid. The rapidity of the acid 
production is variable with different cultures. It is so rapid in 
some cases that, if the specimen be placed at body heat, it will 
curdle in six hours. With other cultures, the curdling under similar 
circumstances would not occur for three days; with some cultures 
curdling never occurs. All specimens of milk, however, become 
acid, although not always sufficiently to precipitate the casein. 
Between these extremes every conceivable grade may be found in 
cultures that are, in other respects, identical; and they represent, 
doubtless, one type, differing in its power of producing acid. 
To this type belongs the largest number of bacteria known to 
cause the souring of milk. Most of the butter starters and cheese 
starters (see page 189) belong to this general class. But the name 
represents a type rather than any single organism. In other words, 
B. acidi lactici represents a group of closely allied varieties. If we 
are asked whether it represents a species or a collection of species, 
we must answer that no one knows what is meant by the term 
species among bacteria. The term species, whatever its significance 
among higher animals and plants, seems to have no meaning among 
bacteria. It is impossible, therefore, to say whether Bact. lactis 
acidi is a single species or a group of species; and we may be content 
simply to recognize under this name the group of lactic acid bacteria 
which most commonly cause milk souring and which comprise 
varieties that, while agreeing in most respects, have slightly differing 
characters. 
The type of milk curdling produced by this organism is quite 
easily recognized. The milk becomes strongly acid, and turns 
into a hard curd, without any trace of gas bubbles, and without the 
separation of whey: it has a clean, sharp taste, and no odor (Fig. 
31, b). This type of curdling has been recognized as a desirable 
one by the dairyman, since it is most favorable for dairy processes 
and is consistent with the production of the best grades of butter 
and cheese. This organism grows readily at temperatures from 
60 to 100 F., growing more rapidly at higher temperatures. At 
a temperature of about 70 it grows with great rapidity, and at 
this temperature it seems to be more vigorous than any other 
