THE RESULTS OF BACTERIAL COUNTS IN MILK 499 



acid colonies, however, compared with the total colonies developed is 

 smaller at 20 C. than at 37 C. Heinemann and Glenn, therefore, 

 favor an incubation at 20 C. for three days over one at 37 C. for two 

 days, and they think that dextrose-litmus-agar is better than lactose- 

 litmus-agar, because more acid producers develop on the former than 

 on the latter. For incubation at 20 C. they used an ice-chest kept 

 cool by circulating tap water and heated by an incandescent electric 

 light connected with a thermoregulator, which disconnected the current 

 whenever the temperature rose above 20 C. 



Interpretation of the Results of Bacterial Counts in Milk. The 

 question of the number of bacteria found in milk is, of course, not 

 alone of theoretical but also of great practical interest. Milk con- 

 taining a great number of bacteria may be unwholesome, and in that 

 case should be condemned and exluded from use as a food. Several 

 cities in the United States have fixed an arbitrary standard of maxi- 

 mum count beyond which milk shall be condemned. It appears, 

 however, that the consensus of opinion among authorities on the 

 question of milk hygiene is that milk cannot be judged from a mere 

 bacterial count as to its fitness or non-fitness for human consumption. 

 Rosenau's opinion as to the significance of large numbers of bacteria 

 in milk has already been given; others have expressed similar or 

 even more pronounced views. 



Rodgers and Ayers, for example, state: "Numerical bacteriological 

 standards which are unquestionably of value are necessarily arbitrary, 

 and are based on the count of total bacteria only. Special methods 

 are necessary to obtain any insight into the relative numbers of 

 bacteria of the different groups occurring in milk, and by the infor- 

 mation thus obtained to form an opinion regarding the cleanliness 

 and care observed in producing and marketing the milk. ... A 

 count of the total bacteria does not always give a true indication of the 

 conditions under which milk is produced. In order to interpret results 

 intelligently it is necessary to know, if possible, the age of the milk 

 and the temperature at which it has been held. Clean milk which 

 has been held several hours in a warm place may contain more 

 bacteria than dirty milk when fresh or even after two or three days 

 if it has been held at a low temperature." 



Swithinbank and Newman, after giving figures of bacteria found 

 in milk in various cities, say: "Many similar investigations with 

 very similar results might be quoted, but the above will suffice to 

 convey an impression of the bacterial contents of many milks. It is, 

 of course, needless to add that quantitative records, whether repre- 

 sented by high or low figures, are in no sense an exact index as to the 

 injurious nature or otherwise of the milk in question, or as to its 

 value for human consumption. A knowledge of the exact quality of 

 the milk, of the kind of organisms and their role, is necessary before 

 any valid conclusions can be drawn. . . . The fact is that numeri- 

 cal estimation of organisms is not, by itself, a sufficient criterion. 



