THE BACTERIOPHAGE AND THE BACTERIUM 83 



tively. Three types of colony may develop, each presenting 

 individual characteristics. 



a. The colonies may be those of normal dysentery bacilli. 4 

 These are encountered especially when working with mixed 

 cultures derived from the inoculation of a suspension with a 

 bacteriophage of low virulence. But even with a very virulent 

 bacteriophage all of the colonies may appear quite normal. 



b. Rare colonies, formed only of cocci, and cultivable under 

 this form. They may be grown in bouillon, where an abundant 

 culture of homogeneous turbidity is secured, or on agar, where 

 the colonies appear somewhat different macroscopically from 

 those of a normal bacillus, being more convex and more opaque. 

 Subcultures obtained by the inoculation of these colonies are not 

 mixed cultures; they contain only cocci, no ultramicrobes being 

 present. The coccoid form is maintained during a number of 

 generations and then the bacterium gradually reassumes its 

 normal form. 



c. The colonies may be mucoid, refractile, difficult to dissociate, 

 and of very diverse size — from the limit of visibility up to those 

 with a diameter of about one millimeter. These colonies are 

 cultivable on agar and reproduce colonies of the same form. 

 They are mixed colonies, and in them the simultaneous presence 

 of both elements, bacterium and bacteriophage, can always be 

 demonstrated. 



Even when abundantly seeded upon agar these colonies never 

 give a smooth layer of growth but always isolated colonies, more 

 or less abundant, and always of variable size. Among the bac- 

 teria of the inoculum but few are able to form colonies. There is 

 always a state of unstable equilibrium between the two elements 

 present : the bacterium with its resistance, and the bacteriophage 

 with its virulence. The bacterium forms, or does not form, a 

 colony according to the accidental predominance of one or the 

 other of these factors. This is especially to be observed when 

 agar is seeded with the agglutinated masses, for however abundant 

 may have been the planting only very rare isolated colonies, all 

 of the mucous type, develop. 



4 B. dysenteriae Shiga is simply taken as an example; all other bacteria 

 give mixed cultures and mixed colonies showing quite similar appearances. 



