RESISTANCE OF BACTERIA 187 



resistance there occurs a multiplication of the bacteria which are the 

 more apt to acquire a resistance (d'Herelle^^^). 



This makes it clear why it is absolutely necessary, during a series of 

 passages made to enhance the virulence of a bacteriophage, to separate 

 the bacteriophage corpuscles in the process of increasing their virulence 

 from the bacteria which have acquired a resistance. As we know this 

 can be done either by filtration or by heating. By causing in each new 

 passage the corpuscles whose virulence is gradually being increased to 

 react upon fresh, normal bacteria which have never yet been in contact 

 with the bacteriophage, the phenomenon of gradually increasing resist- 

 ance does not intervene to counterbalance the progressive acquisition of 

 virulence. 



We have already refuted the rather peculiar theory of Seiffert^^^ 

 according to which it is not the bacteriophage which increases its viru- 

 lence but rather the bacterium which adapts itself to the secretion of a 

 "lysin." An adaptation can not be transmitted to non-existent 

 descendants. As a matter of fact the result of the adaptation of the 

 bacterium to the bacteriophage is exactly the opposite to the production 

 of a "lysin," for the bacterium adapts itself to resist the agent which 

 provokes its dissolution. 



Bacteriophagy is in reality a very complex phenomenon. The two 

 cardinal factors which come into play are the bacteriophage corpuscles 

 on the one hand with their virulence and on the other, the bacterium 

 with its capacity to resist. Each of these factors is by its nature a 

 variable, subject to the conditions of the moment. The virulence 

 and the resistance fluctuate continually; they are increased or they are 

 diminished. The macroscopic result of bacteriophagy, that is to say, 

 the dissolution of the bacterial cells, is the resultant of the two factors 

 which operate in opposition to each other (d'Herelle^'^). 



An experiment of Gratia-"^^ exteriorizes, one might say, this struggle 

 between virulence and resistance. Working with B. coli he has shown 

 that in an acid (pH. 6.8), neutral (7.0), or even shghtly alkaline (7.2) 

 medium one may observe a succession of waves of growth and of dissolu- 

 tion of the bacteria exposed to the action of a bacteriophage. With each 

 wave the growth is a little more accentuated and the dissolution follow- 

 ing is less complete. With the same bacteriophage but under condi- 

 tions which were more favorable, that is, in an alkaline medium (pH 

 7.5), the phenomenon of bacteriophagy occurred normally. 



If the conditions are still more unfavorable to the bacteriophage, for 

 example, if the medium is definitely acid, the bacteriophage no longer 



