THE BACTERIA FOUND IN MILK 39 



second rotating it to the left, ^acid (laevolactic acid), while the third 

 is inactive or racemoid lactic acid, r-acid. The kinds of acid pro- 

 duced in milk have been investigated by Gadamer and more recently 

 by Heinemann.'^ The latter shovt^ed that milk naturally soured at 

 room temperature contains chiefly c?-acid. Milk soured at 37° C. 

 contains chiefly r-acid, with Z-acid in excess if allowed to stand 

 several days. Streptococcus pyogenes and Streptococcus lacticus produce 

 the same kind of lactic acid, i.e. d-VioiA. B. lactis aerogenes, on the 

 other hand, produces Z-acid. The kind of lactic acid produced in 

 naturally soured milk varies, therefore, according to the kind of 

 lactic acid bacillus mainly present, according to the temperature at 

 which the milk has been kept, and according to the length of time 

 the fermentation has lasted. 



It is well known that in the souring of milk under ordinary 

 circumstances a considerable number of other bodies in addition to 

 lactic acid are produced. Amongst these may be mentioned acetic, 

 butyric, and succinic acids, carbonic acid gas, hydrogen, and alcohol. 

 Streptococcus lacticus produces almost pure lactic acid (c?-acid), while 

 B. lactis aerogenes forms, in addition, other substances, volatile 

 acids, ethyl alcohol, etc. 



The lactic acid bacilli {B. lactis acidi type) may be con- 

 sidered from the economic, therapeutic, and public health points 

 of view. From the economic standpoint they are of the 

 utmost importance, since they are active partners in the pre- 

 paration of butter and cheese, but as this is not germane to 

 the present book, this aspect need not be considered. Equally 

 the therapeutic aspect need not be discussed here, although, 

 as is well known, Metchnikoff and his followers have laid great 

 stress upon the value of soured milk as a therapeutic agency. 



Their public health significance is negative rather than 

 positive, since they certainly are not harmful organisms to man. 

 They claim some consideration however, since, in the first place, in 

 enumerating the number of bacteria in milk if low temperature 

 counts are used, they will be enumerated, and so affect the 

 question of a bacterial standard for milk. In the second place, 

 we require to know their relationship to the growth of patho- 

 genic bacteria in milk. As far as our rather incomplete know- 

 ledge takes us it would seem that they and their products are 

 directly inimical to the growth of pathogenic bacteria in milk, 

 so that in themselves they are beneficial. 



Also by producing gross changes in milk which is stale 



1 Journ. of Biological Chemistry, 1907, vol. ii. No. 6, p. 603. 



