LABORATORY WORK IN DAIRY BACTERIOLOGY 28l 



From the fact above stated it will be evident that for a gen- 

 eral determination of the number of bacteria in samples of milk 

 whose origin and age are unknown, it is easier to use agar 

 with a dilution of 1,000 than gelatin. If we have an approximate 

 idea of the number of bacteria, so that the proper dilution may 

 be determined, the use of litmus gelatin will enable us not only 

 to determine in numbers, but will give the means of fairly well 

 separating the different kinds of bacteria. Gelatin, therefore, 

 will give most of the information that agar gives, and consider- 

 ably more under conditions where gelatin can satisfactorily be 

 used. The numbers obtained from plain gelatin plates are 

 usually higher than those obtained from litmus gelatin, and the 

 numbers from either plain gelatin or litmus gelatin are much 

 higher than the numbers from agar. 



Determine the numbers of colonies in both the plain gelatin 

 plates and the litmus gelatin plates. Isolate several colonies as 

 in the following: 



*No. 12. Isolation of Colonies. The bacteria which develop 

 on plates prepared as above described, produce clusters, as al- 

 ready mentioned, called colonies. In many cases it is pos- 

 sible to tell the type of bacteria from the shape and appear- 

 ance of the colony. A study of the plates, especially 

 the litmus gelatin plates, will, therefore, frequently enable us 

 to determine, not only the numbers, but something concerning 

 the kinds of bacteria. The bacteria are now to be isolated from 

 the plates. Sterilize a platinum needle (Fig. 79) in a flame, 

 then dip the tip of it into one of the colonies. When it is re- 

 moved, a number of the bacteria will adhere to it, and these 

 can then be transferred to one of the slant agar tubes, by re- 

 moving the cotton plug and drawing the tip of the wire gently 

 over the surface of the agar. Sterilize the needle again before 

 laying it down. Label the tube by some number, recording in 

 a note book the appearance of the colony, and its source. 



*No. 13. Purification of Cultures. Presumably the culture 



