RESISTANCE OF BACTERIA 215 



On glycerin potato (prepared as for the cultivation of B. tuberculosis) : 

 very rare colonies on the potato, very abundant growth in the fluid at 

 the bottom of the tube. 



In milk: no coagulation in ten days. 



In litmus milk: becomes a mauve color after two months. 



On coagulated serum: no growth. 



In neutral red : no change in two months, either on agar or in bouillon. 



In litmus milk (Petruschky) : acid after ten days and remains acid. 



When the mucous colonies are suspended and heated to 60°C. they 

 are not cultivable, for then the culture contains only the living very 

 virulent corpuscles which are not killed until a temperature of about 

 75°C. is reached. Reinoculated into bouillon, the refractile, mucous, 

 mixed colonies yield two types of culture, (a) mixed cultures showing 

 changes in turbidity, and (6) agglutinated cultures, which, as we know, 

 always depend upon the degree of virulence of the bacteriophage and 

 the capacity of resistance of the bacterium, factors which regulate the 

 appearance of the culture. 



We have seen that if an agglutinate, taken from a mixed culture in 

 stable equilibrium, is introduced into a suspension, a dissolution of the 

 suspension is followed by a growth of the agglutinate. The same thing 

 transpires if an abundant seeding is made on tubes of slant agar having 

 a growth of the Shiga bacillus. First, plaques appear, and then after 

 three or four days a mucous colony develops in the center of each 

 plaque. In both instances the bacteriophage acts upon the normal 

 non-resisting bacteria and dissolves them, then the refractory bacilli 

 multiply as they would have done on sterile agar or in bouillon 

 (d'Herelle^'^i). 



9. THE CAUSE OF SECONDARY CULTURES 



When a secondary culture in bouillon of any bacterium whatever is 

 spread upon agar, the appearance of the agar after incubation may vary 

 greatly, the aspect depending upon the virulence of the bacteriophage 

 found in the secondary culture. When the bacteriophage is very 

 virulent, transfers to agar usually remain sterile, in spite of the fact 

 that the liquid medium may be cloudy and microscopically the material 

 may show many bacterial forms. 



To what may we ascribe this lack of growth on agar when the 

 resistant bacterium develops perfectly in bouillon? We must admit 

 that in a liquid medium the bacteriophage is inhibited, while upon agar 

 it manifests its action. In other words, in bouillon the resistance of the 



