RESISTANCE OF BACTERIA 241 



infra-visible, filterable bacterial forms are ordinarily the cause of the 

 failure occasionally experienced in attempting to increase the virulence 

 of certain bacteriophage races by the method of passages (Tomaselli^^^) . 



11. The ubiquity of the bacteriophage in nature and the frequency 

 of mixed cultures in the animal body (d'Herelle^^^) makes it a priori 

 certain that there are mixed cultures among laboratory stock cultures. 

 That such cultures exist is a fact (Bail^^). Bacterial strains in mixed 

 cultures are, however, relatively rare as has been shown by the careful 

 investigations of several workers. The studies of Flu2i2,2?o ^^q particu- 

 larly interesting because of the number of strains examined and the 

 manner in which he carried out his tests. Of 53 strains of different 

 bacteria he found 51 to be ultrapure. Two only contained a bacterio- 

 phage. 



12. These natural mixed strains are "contaminated" by bacteriophage 

 corpuscles in the same way as are artificial mixed cultures. This is proved 

 by the fact that it is easy to purify them by the method of colony isola- 

 tion on agar as usually employed in bacteriology (d'Herelle^^^) . Strains 

 derived from ultrapure colonies are henceforth ultrapure and it is 

 impossible to derive from them any "lytic principle" whatsoever. 



13. On the other hand it is also possible to isolate bacteriophage cor- 

 puscles from a naturally mixed strain and this can even be done with 

 young cultures (Flu^^^). 



The fact that from all naturally mixed cultures it is possible to obtain, 

 on one hand, bacterial strains henceforth ultrapure, that is to say, not 

 admixed with the bacteriophage, and on the other hand, bacteriophage 

 corpuscles free of the bacteria, shows beyond any possible doubt that in 

 such strains the bacteriophage corpuscles represent an impurity in the 

 bacteriological sense of the word. 



