566 S. HENRY AYERS AND COURTLAND S. MUDGE 



merit on page 5 of the 1916 Standard Methods that "The standard 

 methods, for example, are not such as give a proper count of 

 lactic acid bacteria." In this connection we must say that we 

 see no reason for not taking into account the lactic-acid bacteria. 

 They are of as much significance in tracing the history of a 

 sample of milk as any other type of bacteria, and especially so 

 when their proportion to some other type, such as peptonizers, 

 can be determined. Furthermore, it is believed, since extract 

 agar does not readily support the growth of certain types of 

 lactic-acid bacteria, that this is the most important reason for 

 variations in counts observed when the new standard medium 

 is used. There are certain types of lactic-acid bacteria which 

 will grow slowly on extract agar, and their colonies may be 

 either just visible or not show at all after forty-eight hours' 

 incubation at 37°C. 



In our opinion, a medium should be used (1) which will be 

 relatively cheap, (2) which will give the highest count with 

 colonies large enough to be readily counted, and (3) which will 

 give different pictures with different milks; that is, it should be 

 qualitative so far as possible. If bacteria are in milk there is 

 no logical reason for counting only a portion of them. A bacterial 

 count is made to determine the number of bacteria in milk, and 

 if one medium shows higher counts than another, it is evident 

 that the highest count comes closest to the actual number of 

 bacteria in the milk. With either the infusion or extract agar 

 medium nothing but a simple count can be obtained. If a 

 medium can be devised which gives an indication of different 

 groups of bacteria or even only of a single group, in other words, 

 a medium which will give any information relative to the bacteria 

 in milk besides a simple count, it should prove to have distinct 

 advantage over the media now in common use. 



GENERAL SCOPE OF THE WORK 



In 1911, one of us (Ayers (1911)) described a casein medium 

 which was adapted to the bacterial examination of milk. On 

 this casein medium peptonizers could be readily detected and 



