120 THE ENUMERATION OF BACTERIA IN MILK 



pared in 74 tests. In 28 tests ordinary agar gave the best re- 

 sults, whey agar in 24 tests, and lactose agar in 22 tests. They 

 found that whey agar favoured the growth of lactic acid organ- 

 isms and ordinary agar of organisms other than lactic acid 

 producers. 



Agar and Gelatine. Litmus lactose agar at 37 C. was com- 

 pared with litmus lactose gelatine at 20 C. in 25 tests: of these 

 gelatine gave higher results in 18 tests and agar in 7. Where 

 gelatine showed the higher count the percentage difference was 

 much greater than where agar showed the higher numbers. It 

 was also found that the differentiation of species was much 

 better on gelatine but that there was a considerable loss of 

 plates with this medium. 



Both media were used at 20 C. in 24 tests and in this series 

 gelatine was the better in 14 and agar in 10 samples. When 

 beef peptone gelatine at 20 C. with seventy-two hours incu- 

 bation was tried against beef peptone agar at 37 C. with 

 twenty-four hours incubation, gelatine gave the higher count 

 in 18 tests, agar in 4 tests, and in one test they gave identical 

 results. The total gelatine count, however, was more than 

 double that on the agar plates. The standard method for the 

 examination of milk as adopted by the American Public Health 

 Association in 1912 was the plate method with a plain agar 

 medium of +1.5 per cent acidity, made with beef infusion 

 and 1 per cent each of peptone and dried agar. The 1916 

 report recommended certain alterations; concentrated beef 

 extract, 3 gms. per litre, was substituted for beef infusion and 

 the acidity was reduced to +1.0 per cent: the quantity of 

 peptone was reduced to 5 gms. per litre and the agar in- 

 creased to 1.2 per cent of the dried material. Although the 

 author has not compared fresh beef infusion media with similar 

 media prepared with Lemco for the enumeration of bacteria in 

 milk, his experience with water was that the Lemco media in- 

 variably gave higher and more consistent results. The reason 

 for variable results with beef infusions or decoctions lies in the 

 difficulty in obtaining solutions of even approximately con- 



