156 J. M. SHERMAN AND W. R. ALBUS 



uct from a large number of different dairy farms and so might 

 well be considered as representative of market milk or cream of 

 ordinary quality. After collection the milks were allowed to 

 stand at laboratory temperature until the acidity began to rise. 

 When the acidity reached about 0.2 per cent lactic-acid the 

 samples were plated on lactose agar in dilutions of 1/10,000,000 

 cc. The use of such a high dilution should result in the isolation 

 of the organisms which are taking the most active part in the 

 acid fermentation. The plates were incubated at 37°C. for two 

 days, after which small, dense colonies surrounded by a hazy 

 precipitate^ were fished off into sterile milk tubes. Five colonies 

 were selected from each sample making a total of fifty cultures 

 of this type. Without exception the bacteria selected in this 

 way produced an acid fermentation with the formation of a 

 smooth homogeneous curd. No consideration was given to 

 morphology in the selection of these cultures. The cultures of 

 this group were numbered from 1 to 50 inclusive, and after 

 being tested for purity, were added to the collection. 



Cultures representing the pyogenic type of streptococci were 

 obtained as follows: Samples were taken from the individual 

 cows of the Pennsylvania State College dairy herd by drawing 

 the milk directly from the udder into sterile flasks. Samples 

 of the mixed milk from the College herd were also obtained. 

 This milk is of a very high grade, being produced under the best 

 of sanitary conditions, and its flora is consequently made up 

 chiefly of the types of bacteria which come from the udder. 



These milks were plated in dilutions^ of 1/100 on lactose agar 

 and incubated at 37°C. for two days. Colonies similar to those 

 of the lactic-acid organisms mentioned above were transferred 

 to glucose broth. Cultures which formed chains in broth were 



1 Bacteriologists engaged in milk studies are familiar with the precipitate 

 formed around the colonies of the lactic-acid bacteria when grown on agar made 

 with Witte's peptone and containing a fermentable sugar. When grown on 

 media made with some of the American brands of peptone this characteristic is 

 not seen. Witte's peptone was used in the preparation of the agar from which 

 these cultures were isolated. 



2 In the case of cow 459, which had garget, the milk was plated in a dilution of 

 1/1,000,000 cc. 



