THE BACTERIOPHAGE AND THE BACTERIUM 71 



Up to the present time we have considered only the case where 

 lysis was complete and permanent, and it has been specifically 

 stated that the phenomenon assumes this form only when the 

 bacteriophage possesses a maximum virulence and acts upon a 

 limited quantity of suspension 10 to 20 cc. In spite of the ful- 

 fillment of these conditions it sometimes happens that a suspension 

 which has been lysed in a normal manner with a perfectly limpid 

 appearance, will some days later become turbid. Microscopic 

 examination shows that the turbidity is due to multiplication of 

 the bacteria, and tests of the biologic activity prove that this cul- 

 ture is composed solely of bacteria of the same species as was used 

 in preparing the suspension upon which the bacteriophage was 

 acting. According to the virulence of the strain of the bacteriophage 

 being tested the number of tubes in which this reaction takes 

 place, that is, the development of this secondary culture, 1 is more 

 or less great. 



Inoculations on agar or in bouillon of lysed suspensions, in which 

 secondary cultures later develop, remain sterile up to the time 

 that the secondary culture is formed. This does not often occur 

 until five or six days after the lysis, sometimes even later. 



Experiment XVII. A suspension of Shiga bacilli, containing 250,000,000 

 bacilli per cubic centimeter, is inoculated with 0.001 cc. of a culture of the 

 bacteriophage. Normal lysis takes place in five hours, with the medium 

 perfectly limpid. The lysed suspension is planted on agar and in bouillon 

 1, 2, 3, 4, 5, 6, and 7 days after the lysis is completed. All the plantings 

 remain sterile. On the eighth day the lysed suspension is slightly clouded. 

 On the ninth day a drop is inoculated into broth and on to three tubes of 

 agar. Two of the agar tubes remain sterile, the third shows four small 

 colonies. The broth tubes give a culture agglutinated in the sediment. 



The resistance of diverse strains of a single bacterial species 

 is not constant. Each strain appears, on leaving the organism, 

 to be possessed of an individuality which is rapidly effaced by- 

 successive cultivations upon an artificial medium. 



1 In order to facilitate exposition I have called a "secondary culture" 

 one growing again in a lysed suspension ; a "mixed culture," the inoculation 

 into a nutritive medium of a "secondary culture" with the coexistence in 

 the medium of bacteria and bacteriophage. 



