IDENTITY OF THE LACTIC-ACID STREPTOCOCCUS 155 



recently been demonstrated (Sherman and Hastings, 1914; 

 Evans, 1916) that a large proportion of healthy milch cows 

 harbor streptococci within their udders which are morphologi- 

 cally and culturally identical with the typical Strept. pyogenes 

 of mammitis and other pathological conditions. Hence such 

 organisms are probably always present in milk produced from 

 any considerable number of animals. 



From all of these facts it would appear that there are two main 

 groups of streptococci in milk, the true lactic-acid organisms and 

 streptococci of the pyogenes type whose chief source is the 

 udder. The lines of cleavage between these groups should 

 certainly be established, and it was with the object in view of 

 finding points of differential value that the present study was 

 undertaken. 



SELECTION OF CULTURES 



The ordinary way in which to attack a problem of this kind 

 would be to isolate a large number of miscellaneous chain-form- 

 ing cocci from various grades of market milk and milk products 

 and to subject them to a detailed study with the intention of 

 separating them in that way into their natural divisions. Our 

 manner of approach has been different in that the cultures were 

 obtained in such a way as would tend to limit the collection to 

 organisms typical of the groups under consideration, and not to 

 include a great number of closely related types which would 

 complicate the study and make more difficult their separation 

 into natural groups. Since the true lactic-acid organism has 

 not been defined so as to enable it to be distinguished from other 

 closely related bacteria, it is obviously impossible to tell just 

 which are and which are not typical lactic streptococci. It 

 would seem, however, that the organisms which take an active 

 part in the natural souring of dinorary market milk could be 

 properly designated as true lactic-acid bacteria. Proceeding on 

 this assumption, cultures were isolated by th€ following method : 



Ten samples of raw milk and cream were obtained from the 

 receiving vat of the Pennsylvania State College Creamery on 

 as many different days. These samples were of the mixed prod- 



