RESISTANCE OF BACTERIA 



203 



These experiments are of great interest for they indicate, as Gratia 

 himself has remarked, a method of increasing the virulence of a bacterio- 

 phage and they show how virulence toward diverse bacterial species may 

 be acquired. 



6. THE LOSS OF RESISTANCE 



Bordet and Ciuca^^ were the first to show that the serum of an animal 

 which had received a series of injections of a bacteriophage suspension 

 possesses the property of inhibiting the action of this bacteriophage. 

 We will return to these experiments in another chapter. They dis- 

 tributed^- over the surface of an agar slant a few drops of an anticoli- 

 bacteriophage serum, and allowed the agar to become impregnated with 

 this by holding the tube for a few hours in the incubator. They then 

 seeded the tube with a resistant B. coli admixed with a bacteriophage. 

 The latter was unable to develop and consequently they obtained an 



ultra-pure culture of B. coli refractory to bacteriophagy. By a series of 

 subcultures they showed that the resistance was gradually lost, but it 

 was only after 21 consecutive passages carried out in the complete 

 absence of the bacteriophage that its sensitivity was restored to a degree 

 corresponding to that of the original B. coli. 



Bruynoghe^^^ did not succeed in purifying a contaminated strain of 

 B. dysenteriae Shiga with an antiserum. He demonstrated that bacterial 

 "recovery" and the return to sensitivity is the more difficult to obtain 

 when bacteria have been for a long time and during a very considerable 

 number of generations, in contact with bacteriophage corpuscles. 



Ehava and Pozerski^^i have noted that if the material resulting from 

 the complete dissolution of a suspension of dysentery bacilli by a 

 bacteriophage of high virulence (but not maximal for then the agar 



