192 ISOLATION OF BACTERIA IN PURE CULTURE 



two kinds of bacteria stick together so closely that even the 

 separate colonies contain both organisms. This is not 

 common, however. The plate colonies frequently develop 



Fig. 128.— Dilution plates. X ro- 1. shows the first dilution, the colo- 

 nies are so numerous and small that they are invisible (compare Fig. 129); 

 2, shows fewer and hence larger colonies, but too crowded to isolate (compare 

 Fig. 130) ; 3, shows the colonies larger and well separated, so that it is easy 

 to isolate from them (compare Fig. 131). 



from groups of bacteria which were not separated, but as 

 these are of the same kind the culture is essentially pure. 



Fig. 129.— a portion of plate 1 in Fig. 128 as seen under the low-power 

 objective. X 100. Very small, closely crowded colonies. 



Another method which is frequently applicable with 

 material from human or animal sources is to (3) rub the 

 material over the surface of a slope tube or of medium solid- 



