150 MANUAL OF BACTERIOLOGY. 



1-1,000,000. The i-io, dilution should be prepared by 

 shaking the sample twenty-five times and taking i c.c. and 

 putting it into 9 c.c. of sterile water. The i-ioo, dilution is 

 prepared in the same way, adding i c.c. of the milk to 99 c.c. 

 of sterile water. The other dilutions are prepared from the 

 i-iooo dilution. The plate giving from 40 to 400 colonies 

 should be selected for counting. All the colonies on the plate 

 should be counted in preference to counting any part of a 

 plate and calculating the total number. The culture medium 

 should be agar, containing i per cent, agar, and it should 

 be made from the watery extract of beef. The reaction should 

 be + 1.5,* American Board of Health Scale. The majority of 

 those consulted by the committee count the colonies after 

 twenty-four hours, incubation at 37 C. in a moist incubatcr. 



As a routine procedure, in cold weather, entirely satisfac- 

 tory results may be obtained by taking i c.c. of the milk to be 

 examined after it is thoroughly mixed, and putting it into 9 c.c. 

 of sterile water, and taking i c.c. of this solution in 9 c.c. of 

 sterile water. Plates made from this dilution with i-io c.c. and 

 with i c.c. respectively have been found to give closely corre- 

 sponding results, and unless the milk is badly contaminated it 

 is always possible to count the colonies readily. In warm 

 weather and in the case of cream, a third or even a fourth dilu- 

 tion should be made. Where the milk or cream are mixed 

 with the medium in the tube, the resulting colonies are apt to be 

 more uniformly distributed on the plate than where the milk is 

 put into the Petri dish and the culture medium poured in after- 

 ward. The number of bacteria remaining in the test-tube 

 in the former method of proceedure must be very few where 

 the medium is properly fluid when it is poured. The ob- 

 jection to this method would appear to be purely theoretical, 

 and counterbalanced by its -advantages. 



Contamination of the milk may occur from the outer surface 



*See page 67. 





