82 THE BACTERIOPHAGE 



and the passage of the parasitic agent from one bacterium to 

 another is readily accomplished since a strong chemotactic force 

 is not required to bring together the two organisms in the struggle. 

 When a culture develops on agar, the appearance of the growth 

 may show considerable variation, determined by the relation 

 between the resistance of the bacterium and the virulence of the 

 bacteriophage. These two factors being inherently variables 

 afford an infinite number of possible combinations, resulting in an 

 infinite variation in the possible appearances on agar. At first 

 sight, one of two principal distinguishing aspects may be present : 



1. A smooth layer of culture, always located in the upper portion 

 of the agar slant where the medium is less thick (although, it is 

 needless to say, the mixed culture may be distributed over the 

 entire surface of the slant) . The extent of this covering layer is 

 variable. Sometimes there is only a fringe of bacterial growth 

 at the extreme upper margin of the medium, the remaining portion 

 being sterile. At other times the culture layer covers one-tenth, 

 one-fourth, one-third, one-half, or even three-fourths of the slant, 

 the portion remaining sterile always being the lower section of 

 the slant where the agar is of greatest depth. The cause of this 

 is simple. We have seen that the colonies of the bacteriophage 

 appear as circular plaques, apparently sterile, and that they are 

 of greatest size where the agar is deepest. The reason for this 

 has been stated, and the present instance is but an application of 

 this general fact. 



Certainly most bacteriologists will recall having seen cultures 

 of this character without having recognized the cause. As a 

 matter of fact, many cultures are to be found among culture 

 collections which are in reality nothing but mixed cultures. In 

 all cases, when one observes abnormal cultures, presenting the 

 characteristics which have been described, one may be sure that 

 such a mixed culture is present, that is, the culture is one which 

 is infected with the bacteriophage. 



2. The culture may consist of more or less numerous isolated 

 colonies, even when the medium has been abundantly inoculated. 



These isolated colonies, in their turn, may present different 

 appearances, associated always with the degrees of resistance 

 and of virulence of the bacterium and the bacteriophage respec- 



