VIRULENCE OF THE BACTERIOPHAGE 159 



4. VARIABILITY IN THE VIRULENCE OF THE BACTERIOPHAGE 



From the time of Pasteur we have known that in a single bacterial 

 culture each of the bacteria possesses certain characters which are pe- 

 culiar to itseK and differentiate it from the associated organisms, even 

 though all of the organisms present the general characters belonging to 

 the species. Such qualities as vitaHty, resistance to heat and to anti- 

 septics differ for every organism within a culture. This fact is partic- 

 ularly striking as regards virulence. For example, let us take a culture 

 of B. pestis and implant it on an agar slant in such a way as to secure 

 isolated colonies. By the inoculation of susceptible animals we can 

 show that cultures of some of these colonies kill the animal in an in- 

 finitely small amount while others are inoffensive in very large 

 quantities. 



The bacteriophage, just hke the bacterium, is of corpuscular nature 

 and different races of the bacteriophage, just like different strains of 

 bacteria, possess different virulences. One might, indeed, ask if the 

 analogy does not go still further and if in a given suspension of bacterio- 

 phage each corpuscle does not possess a particular virulence. It is 

 very easy to determine that this is indeed the case. 



Isolate, by means of dilutions, an extremely active Staphylo-bacterio- 

 phage, such a race as I have described in the preceding section. Dis- 

 tribute 10 cc. of the Umiting dilution among 10 suspensions of the 

 staphylococcus, 1 cc. to each. The results of such an experiment will 

 be as follows: 



After 36 hours : 1 suspension will be clear 



1 suspension will be clouded 

 8 suspensions will be turbid 



After 48 hours : 2 suspensions will be clear 



2 suspensions will be clouded 

 6 suspensions will be turbid 



After 72 hours : 5 suspensions will be clear 

 1 suspension will be cloudy 



1 suspension will be very cloudy 



3 suspensions will be turbid 

 After 7 days: 5 suspensions will be clear 



2 suspensions will be very cloudy 



3 suspensions will be turbid 



Spreadings made upon agar, and repeated passages will show that the 

 3 turbid suspensions are free of bacteriophage; that is, bacteriophage 



