142 Report of the Bacteriologist of the 



process was repeated and a small fraction of a c. c. from the second 

 dilution was added to the culture medium. 



Cultures were made in Petri dishes having an internal diameter 

 of 91-92 mm. For the sake of convenience in counting and to 

 prevent the inhibiting effect of closely-crowded colonies, the aim 

 was to so arrange the dilution that the growth would be about 

 500 colonies to the plate. In the case of the pasteurized milk, 

 no dilution was necessary, but a measured fraction of a cubic 

 centimeter was added directly to the nutrient media. 

 • Media used. — The tabulated results given below were all 

 obtained upon lactose agar made neutral to phenolphthalein with 

 sodium hydroxide and containing 2 per ct. lactose and 1.7 per 

 ct. agar. Agar was chosen in preference to gelatin, because in 

 some previous work of a similar nature at the Wisconsin Station 

 it was found that agar at 28° C. (81.5° F.) gave higher numerical 

 results than gelatin at room temperature. Among the substances 

 now available, there seems to be none that will call out all the 

 individual germs when left at any one temperature. 



It is not maintained by the authors that the numbers given 

 below represent the exact number of organisms present either in 

 the pasteurized or unpasteurized milk. All that is hoped for is 

 that they are a close approximation and that having been taken 

 under similar conditions may be found to be directly comparable. 



Incubating tem'perature. — The plates were placed in an incuba- 

 tor at 30° C. (83° F.) and counted at the end of 48 hours. This 

 temperature was beheved to be near the optimum for the growth 

 of most of the germs present and the time was thought to give 

 maximum returns with a minimum amount of error. An expos- 

 ure at higher temperature caused a rapid drying of the plates and 

 one for a longer time did not usually give higher results, while 

 the rapid spreading of superficial colonies made the counting un- 

 certain. 



Growth at room temperature required so much longer time as 

 to complicate the work and the rapid multiplication of certain 

 proteus forms made an accurate count very difficult. 



