24 EXPERIMENTAL DAIRY BACTERIOLOGY 



number of colonies will not appear in less than six or eight 

 days. It is, however, often advisable to secure results before 

 the expiration of such a long period of incubation, and for 

 comparative purposes a shorter time than the maximum here 

 given is frequently used as a basis for official counts. 



Agar on solidifying expresses a slight amount of water. 

 This often collects on the surface of the medium in a thin 

 film, allowing the formation of a spreading surface growth. 

 Frequently a thin, nearly transparent growth may spread 

 between the under surface of the culture medium and the 

 glass. If freshly prepared plates are placed immediately in a 

 37 C. incubator, evaporation from the culture medium occurs, 

 which permits of condensation on the cover of the dish, and 

 the moisture sometimes collects to such an extent that drop- 

 lets of water fall on to the medium, causing the formation 

 of a spreading growth and seriously interfering with subse- 

 quent counting. To obviate this, the plates may be inverted 

 in the 37 C. incubator. Covers made of porous earthenware 

 are also useful in preventing this trouble. 



Counting. To enumerate the developing colonies, the cover 

 is removed from the culture dish and the plate placed on 

 a black surface, which is divided into appropriate divisions, 

 in order to facilitate counting (Frost's modification of the 

 Jeffer's plate counter is most satisfactory). The counting 

 should be done by the aid of a hand lens, magnifying 5-8 

 diameters. When possible, the entire plate should be counted. 

 Where the colonies are too numerous, an estimate may be 

 made by counting a representative portion of the plate, and 

 from the data obtained calculating the total number of colo- 

 nies on the plate. This process is, however, often unsatisfac- 

 tory, especially if the colonies are not uniformly distributed. 

 If the plate was not allowed to solidify in a level position, 



