194 PASTEURISED OR HEATED MILK 



it is sometimes desirable, as in the study of the effect of heat or 

 chemical germicides upon the bacterial flora, to determine the 

 relative proportion of this group to the total bacteria, without 

 reference to the morphological characters of the individual 

 members. 



This division of the flora into groups on an acid-producing 

 basis is necessarily an empirical one, but it is comparatively 

 simple and has proved useful on many occasions. 



The development of this method and its application to the 

 examination of raw and pasteurised milk is largely due to 

 Ayers and Johnson of the United States Department of Agri- 

 culture. In their earlier work they grouped the flora into acid 

 forming, alkaU forming, inert, and peptonising organisms ac- 

 cording to their action on litmus lactose gelatine. This was 

 effected by plating out the sample on this medium and counting 

 the various groups after incubation for five days at 18° C. By 

 this method it is often difficult to distinguish between the feeble 

 acid formers, the feeble alkali formers, and the inert group, but 

 fairly satisfactory results have been obtained with it in the 

 author's laboratory ^ and it has the advantage of being much 

 quicker and simpler than the later developments. The first 

 modification made by Ayers '^ was an effort to obtain a more 

 accurate count of the peptonising group by the elimination of 

 spoilt plates caused by the spread of the gelatine liquefiers. A 

 neutral lactose casein medium (see Appendix) was substituted 

 for litmus lactose gelatine and the peptonisers differentiated 

 by flooding the surface of the medium, after six days incuba- 



N 

 tion at 30° C, with — lactic acid. The colonies of peptonis- 

 ing organisms became white owing to the precipitation of casein 

 by the acid. Ayers, in the same report (p. 227) also suggested 

 the division of the flora into five groups according to the action 

 on litmus milk. The colonies developing on lactose casein agar 

 or infusion agar were fished into litmus milk tubes and incu- 

 bated for fourteen days at 30° C. According to the appear- 

 ance of the milk after this period the organisms were classified 



