THE BACTERIOPHAGE AND THE BACTERIUM 81 



If some of the agglutinate, even if washed, is introduced into a 

 suspension of B. dysenteriae lysis takes place and the suspension 

 becomes perfectly clear within five to six hours. Four or five 

 days later, however, the agglutinated masses begin to appear and 

 gradually increase in size. The ultramicrobes contained in the 

 agglutinate used as inoculum provoke the lysis 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 agglutinated form in the presence of a per- 

 fectly 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 having but little density forming 

 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 organ- 

 isms which are present. 



Mixed colonies on agar 



Instead of seeding the mixed cultures into broth they may be 

 inoculated on to a solid medium. 



Often the agar will remain sterile, indicating that the equili- 

 brium has been disturbed in favor of the bacteriophage. As has 

 been said, this reaction may occur with inoculation into broth, 

 but it is more frequent when agar plantings are made. It appears 

 that the bacteria on agar are more readily attacked than when in 

 a fluid medium. For this there may be several reasons, the prin- 

 cipal one doubtless being the proximity of the bacteria. The first 

 phase of the struggle is certainly associated with the phenomenon 

 of chemotaxis. In a fluid medium the bacteria in suspension are 

 separated by considerable spaces, certainly considerable when 

 compared with the diameter of the ultramicrobe. Upon solid 

 media, on the other hand, the bacteria actually touch each other, 



