20 EXPERIMENTAL DAIRY BACTERIOLOGY 



Each colony represents a living bacterial cell in the sub- 

 stance plated, only when the cells are uniformly distributed 

 throughout the liquid and no clumps of cells persist. This 

 condition is impossible to attain in viscous liquids and in 

 those in which growth takes place ; hence, on culture plates, 

 many colonies represent the progeny of a clump of cells, 

 numbering many score or more, instead of being the result 

 of the growth of a single cell. The results obtained from the 

 examination of any sample will thus vary with the care used 

 in obtaining an even distribution of the bacteria. If a sample 

 of milk is allowed to stand for twenty-four hours and plate 

 cultures prepared before it has been shaken, and another set 

 after the -milk has been shaken for several minutes, the re- 

 sults will vary widely, due to the more or less complete rup- 

 ture of the cell clumps of the bacteria. 



If the colonies on the plates are very numerous, they will 

 consist mainly of those kinds that, under a given environment, 

 find most favorable conditions for growth. Those finding less 

 favorable conditions are inhibited. If lactose-agar plates are 

 seeded with a sample of sour milk and kept at 25- 30 C., 

 the ordinary sour milk or lactic-acid bacteria will grow very 

 rapidly, and the putrefactive bacteria will not develop ; but if 

 the cultures are made with gelatin containing no milk sugar, 

 the sour milk bacteria will develop very sparsely, or not at 

 all, and only the other type,s will be found on the plates. 



If the colonies are less numerous, so that each is not within 

 the zone of influence of other developing colonies, this in- 

 hibition will not take place. Generally such inhibition or 

 "antagonism" is caused by the effect of the soluble by- 

 products of one form diffusing throughout the medium. 



For ideal results each colony should develop as though it 

 were the only colony on the plate. This condition is of course 



