178 MILK AND THE PUBLIC HEALTH 



CHAP. 



as to the best estimation procedure to adopt. The tempera- 

 ture of 37 C. is, in the writer's opinion, preferable to 20- 

 22 C. for incubation of the milk-plates, since at the higher 

 temperature the usually harmless lactic acid bacilli do not 

 grow, or at least not readily, and it is a disadvantage to 

 enumerate these organisms. On the other hand the manurial 

 bacteria, which serve as the measure of manurial pollution, 

 grow best at 37 C. While, therefore, the count at the higher 

 temperature will be lower, it will yet be more valuable. The 

 higher temperature also enables results to be recorded earlier, 

 thus saving time, and avoiding the locking up of a large 

 number of plates. 



It should, however, be mentioned that a number of workers 

 prefer the enumeration at the lower temperature. For ex- 

 ample, Heinemann and Glenn l have recently compared the 

 value of enumerations at 20 and 37 C. respectively. They 

 found that using both glucose litmus agar and lactose litmus 

 agar the number of colonies after 1 day is higher at 37 C., 

 after 2 days is higher at 20 C., and after 3 days' incubation 

 the number of colonies at 20 is about double that at 37 C. 

 They found many more plates lost by the overgrowth of spread- 

 ing colonies at 37 than at 20 & C. They favoured the in- 

 cubation at 20 C. for 3 days chiefly because, in their opinion, 

 more groups of bacteria have a chance to develop than at 

 37 C., and this gives a better insight into the production 

 and handling of milk. 



For ordinary enumerations simple nutrient agar, of 

 + 1 reaction, may be used. This medium may be modified 

 for special purposes. For example, lactose litmus agar and 

 glucose litmus agar have been made use of instead of ordinary 

 agar, since they allow the bacilli which ferment these sugars 

 to be differentiated and roughly enumerated. Heinemann 

 and Glenn, comparing these two media, found that there was 

 in some samples a decided relative decrease of acid colonies 

 after 2 and 3 days at both 20 and 37 C. in lactose agar. 

 This phenomenon, they believe, is due to the fact that some 

 milk bacilli of the B. aerogenes type form red colonies at first, 

 and then later these colonies assume the blue colour again. This 

 change was not observed in glucose agar, and in consequence 



1 The Journ. of Infectious Diseases, 1908. v. p. 412. 



