168 THE BACTERIOPHAGE AND ITS BEHAVIOR 



It is obvious that it is impossible to state any specific rule as applying 

 to the phenomenon in all cases. Each bacteriophage behaves in a par- 

 ticular manner. The most that can be said is that in general races with 

 a high virulence retain virulence for a long time, while a low virulence 

 disappears relatively quickly. 



Antiseptics hkewise exert an effect upon virulence, but we will reserve 

 a study of this subject until we consider the properties of the bacterio- 

 phage corpuscle. 



Virulence is weakened through contact with resistant bacteria. This 

 aspect of the subject will be considered in detail in the next chapter. 



7. HOMOGENEOUS AND HETEROGENEOUS BACTERIAL SPECIES 



Different bacterial species behave in a different manner as regards 

 susceptibility to bacteriophagy. With certain species it is found that 

 when one strain is susceptible to a given race of the bacteriophage all 

 other strains are also susceptible.* 



The Shiga dysentery bacillus is typical of those species which I 

 have termed homogeneous insofar as the bacteriophage is concerned. 

 During the past 10 years I have isolated several hundred races of bac- 

 teriophage active for this bacterium and I have never yet found a strain 

 of the Shiga bacillus which was not attacked by any of these races. The 

 degree of virulence is the only variable. A given bacteriophage of max- 

 imum activity for a single strain may possess only a moderate virulence 

 for others, but a few passages are always sufficient to intensify the 

 virulence for those strains which were at first but weakly attacked. 



B. pestis is another bacterial species which I have found to be homo- 

 geneous as regards bacteriophagy. 



I have shown^i^ that certain races of Typhoid-bacteriophage have 

 an extremely specific action, attacking but a single strain of B. typhosus 

 to the exclusion of all other strains. Bordet and Ciuca^*^ have confirmed 

 this finding, and they have shown that with B. coli a comparable speci- 

 ficity may be demonstrated with some races of bacteriophage. 



My experiments have disclosed the fact that this non-susceptibility 

 of certain strains of a bacterium when others are attacked, is limited 

 to certain bacterial species, and these I have termed heterogeneous. 



* Naturally, this is true only insofar as it deals with typical strains, that is to 

 say, with those presenting all of the characters of the species. In general, atypi- 

 cal strains (those which are arbitrarily^ classified in a species to which they do not 

 conform in all characters) are found to be resistant to the action of the bacterio- 

 phage. We will return to this subject later. 



