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 x 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 1 00 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 



