214 THE BACTERIOPHAGE AND ITS BEHAVIOR 



pension of B. dysenteriae dissolution takes place and the suspension 

 becomes perfectly clear within five or six hours. Four or five days later, 

 however, the agglutinated masses begin to appear and gradually increase 

 in size. The corpuscles contained in the agglutinate used as inoculum 

 provoke the dissolution of the normal bacilli of the suspension, bacilli 

 which are non-resistant, and then later the resistant agglutinated bacilli 

 in their turn reproduce and the result is that which would have been 

 secured had they been inoculated into fresh sterile bouillon. 



All stages intermediary between these two extremes may be obtained ; 

 cloudy mixed cultures presenting the appearance of a normal bacterial 

 culture where the equilibrium is essentially unstable; cultures in agglu- 

 tinated form in the presence of a perfectly limpid fluid, representing a 

 state of stable equilibrium. The medium may be more or less cloudy 

 with the bacterial masses more or less compact, sometimes resembling a 

 coagulum. The type of the mixed culture bears a relationship to the 

 virulence of the bacteriophage and to the resistance of the bacterium. 

 Hence, the appearance of the mixed culture may be as variable as is the 

 variability in the properties of the two organisms which are present. 



I have spoken above of mixed colonies on agar formed by very 

 resistant bacteria and very virulent corpuscles. These colonies are 

 small, mucous, and difficult to dissociate. 



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 bacteria 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. 



The cultures secured by the inoculation of the mucous colonies on 

 different media show the following reactions : 



In agar stabs: small lenticular colonies about the needle track. 



In gelatin: as in agar, the resistant bacteria remain alive and 

 cultivable for at least eleven months. In the case of the Shiga dysen- 

 tery organisms this represents a viability at least ten times as great as 

 that of the normal bacillus. 



In gelatin stabs: large opaque colonies with opaque centers. 



