280 



BACTERIOLOGICAL ANALYSIS OF MILK. 



a large per cent, of the non-acid bacteria,, the milk must be 

 regarded with more suspicion. Two actual analyses of milk 

 may be given in the way of illustration. 



Of these two samples it will be seen that the first contains 

 many more bacteria than the second, but the percentage of 

 lactic bacteria is very great. The second sample contains 

 small numbers but with very small per cent, of lactic bac- 

 teria. The second sample of milk is doubtless fresh, but the 

 first is a perfectly normal milk and not suspicious in spite of 

 its large total numbers. If No. I, with its high numbers, had 

 a large percentage in the third and fourth columns it would 

 have been suspicious. 



This method of testing milk is not given here as by 

 any means complete, nor is it assumed that the results ob- 

 tained will enable us to determine positively the whole- 

 someness of milk. It is believed, however, that such a 

 method of bacteriological study is an advance over the practice 

 of simply counting the numbers, which has been the common 

 method of the past. The great difficulty of the whole method 

 consists in the fact that it sometimes happens that the milk 

 contains many liquefying bacteria, which grow rapidly and 

 liquefy the gelatin. If the gelatin begins to liquefy rapidly it 

 is necessary to study the plate at once, in spite of -the fact that 

 the differentiation is not perfect. If, however, the liquefying 

 bacteria are not numerous and do not grow rapidly, the plates 

 may be kept for six or seven days, or even more before they 



