268 BACTERIOLOGICAL ANALYSIS OF MILK. 



In fresh milk it is quite rare to find the common lactic bac- 

 teria very numerous. If milk is retained at a moderate tem- 

 perature for a number of hours the few lactic bacteria origi- 

 nally present commonly grow rapidly, and soon come to out- 

 number all other species put together. Hence, it follows that, 

 if a sample of market milk contains a large number of bac- 

 teria and of these a large percentage are lactic bacteria, this 

 is probably due to the fact that the milk has been kept at a 

 moderately warm temperature for a number of hours. 



If, on the other hand, the milk should show large numbers 

 of miscellaneous bacteria, either of the putrefactive type or 

 those which have no effect upon the milk, the inference to be 

 drawn is different. We are then led to suspect that there 

 was a greater primary contamination of the milk. If there is 

 much filth around the barn or the cows, the milk is likely to 

 be contaminated with a great variety of bacteria, and hence 

 the presence in a sample of market milk of great quantities 

 of these miscellaneous types suggests an excessive contami- 

 nation in the original milk. Moreover, if we are correct in 

 the assumption that the common lactic bacteria are beneficial 

 rather than the reverse, it is perfectly evident that this quali- 

 tative differentiation of species will be of decided significance 

 in determining the wholesomeness of the milk as an article 

 of diet. 



QUALITATIVE BACTERIOLOGICAL ANALYSIS OF 



MILK. 



This requires different culture media from that described 

 above. It is prepared as follows : 



I. Sugar-gelatin. Mix together the following: 



Water 1,000 c.c. 



Peptone 13 gms. 



Gelatin 150 " 



Milk sugar 30 " 



Liebig's Extract of Beef 7 " 



