THE BACTERIOPHAGE AND THE BACTERIUM 67 



bacterium are duplicated in the struggle between the parasitic 

 bacteriophage and the attacked bacterium. The resemblance is 

 complete. It is only a matter of descending a degree in the order 

 of size in the beings concerned. 



It is also a property of living beings to never be the same at 

 any two moments of their existence. If the phenomenon of 

 serial transmissible bacteriolysis which we are considering were of 

 purely diastatic nature, the activities as they unfolded would 

 follow a fixed plan; for if the active element was not varied in 

 quantity its quality would in all cases be constant. But we have 

 seen that quite the contrary is the case. The phenomenon is 

 independent of the quantity of the active element employed. 

 The dominating feature is the quality of this element. Such are 

 precisely the characteristics of vital activities. A poison acts in 

 accord with its mass; a bacterium, with its virulence. 



Experiment has already shown that a bacteriophage but 

 weakly capable of attacking an organism is susceptible to increase 

 in potency through successive passages in contact with the bac- 

 terium which it attacks. In order to recognize the differences 

 presented between different strains of the bacteriophage it is 

 preferable to work with strains freshly isolated from the organism. 



Experiment XV. (A) . Ten cubic centimeters of a suspension of Shiga 

 bacilli are inoculated with 1 cc. of a filtrate made directly from the feces 

 of a patient with dysentery. The suspension is held at 37C. Counts of 

 the ultramicrobes, made at different times during the incubation, give the 

 following results when 0.01 cc. is plated on agar. 



When plated immediately, there develop 16 plaques, representing 1,600 

 ultramicrobes per cubic centimeter. The filtrate from the feces therefore 

 contained 16,000 per cubic centimeter. 



After one and one-quarter hours, the count is 40 plaques, or 4,000 per cc. 



After two and one-half hours, a 1 :10 dilution gives 42 plaques, or 42,000 

 per cubic centimeter. 



After three and three-quarter hours, a 1 :100 dilution gives 18, or 180,000 

 per cubic centimeter. 



After five hours, a 1:1000 dilution gives 4, or 400,000 per cubic centi- 

 meter. 



After fourteen hours, the lysis is not complete, the medium is cloudy 

 and becomes more and more turbid, so that after forty-eight hours it is very 

 turbid. Here there is an abundant culture, but lysis is never complete. 

 The bacteria have, then, acquired a certain resistance which has allowed 

 them to reproduce in spite of the presence of the bacteriophage. 



