SIGNIFICANCE OF NUMBERS OF BACTERIA. 263 



age the bacterial content of milk proves to be surprisingly 

 bad. It is occasionally stated that the milk of our cities 

 contains more bacteria than the city sewage. This state- 

 ment is certainly true. It frequently happens that the milk 

 distributed in summer weather contains more bacteria than 

 the ordinary sewage, and in many cases the number of bac- 

 teria in the city milk far surpasses that found in the worst 

 sewage. 



It is manifest that we cannot interpret the number of bac- 

 teria in milk in the same way that we do the number in 

 water. We cannot infer that milk is a more unhealthful 

 product than sewage because it contains more bacteria. The 

 use of sewage for drinking would be disastrous ; but milk 

 that contains bacteria in large numbers is constantly used 

 without injury. In short, the determination of the number 

 of bacteria in milk does not determine its wholesomeness. 

 We cannot condemn a sample of milk upon the same bac- 

 teriological grounds which would warrant us in condemning 

 a sample of water. 



In the first place the presence of a comparatively small 

 number of bacteria does not necessarily mean that the milk 

 is wholesome, for that small number may contain certain 

 pathogenic forms, and in this case the milk would be decid- 

 edly injurious. On the other hand, the presence of large 

 numbers of bacteria does not necessarily mean that the milk 

 is unwholesome. We have already seen that there are 

 no reasons for believing that lactic bacteria are injurious, 

 but, possibly, on the contrary, are advantageous to our diges- 

 tive functions. If, therefore, of the large number of bacteria 

 found in a sample of milk most of them are of the normal, 

 harmless, lactic type, their presence in great quantity may 

 not render the milk less wholesome. 



Although we cannot determine the wholesomeness of milk 

 by a quantitative analysis of the bacteria, nevertheless such an 

 analysis is useful. The number of bacteria in milk is depend- 



