CULTURAL TECHNIQUE 23 



in the hand and passing them quickly through the flame of 

 a Bunseii burner. After the culture medium is distributed 

 the plates should be placed on a perfectly level surface to 

 solidify, in order that the culture medium may be uniform 

 in thickness and the colonies evenly distributed. 



Incubation. Nearly all forms of bacteria found in milk 

 grow at room temperature, about 20 C., although higher 

 temperatures hasten the growth of many kinds. 



Two temperatures are ordinarily used in bacteriological 

 laboratories, 20 C. (room temperature) and 37.5 C. (blood 

 heat). In order to maintain a constant temperature an ap- 

 paratus known as a thermostat or incubator is used. A 

 temperature of 37.5 C. is maintained by a gas flame, the 

 gas supply being regulated by a thermoregulator. A constant 

 temperature of 20 C. is difficult to maintain, since under 

 summer conditions room temperature often exceeds this 

 limit. If an abundant supply of cold water is available, the 

 thermostat may be kept cool, i.e. at a temperature below 

 20 C., and then heated with a small, regulated gas flame. 



Gelatin cultures must always be incubated at tempera- 

 tures below 25 C. Agar cultures may be kept at any 

 desired temperature up to the maximum for bacterial growth, 

 which for most species does not exceed 40-45 C. 



The proper time of incubation, i.e. the period between the 

 preparation and the counting of the plates, \vill depend upon 

 the temperature. If 37.5 C. is used, the number of colonies 

 will not materially increase after forty-eight hours incuba- 

 tion. The individual colonies will, however, increase in size 

 and make the counting easier. This is especially true if the 

 plates are incubated for another forty-eight hours after they 

 are removed from the 37 C. incubator. Plates kept at 20 C. 

 should not be counted before three days, and the maximum 



