RESISTANCE OF BACTERIA 211 



A mixed culture results from the establishment of a state of equilib- 

 rium between the virulence of the bacteriophage corpuscles and the 

 resistance of the bacterium. In such cultures a true S3mibiosis obtains 

 in the true sense of the word; parasitism balanced by the resistance 

 to infection.* What is, in the last analysis, the mechanism leading to 

 the formation of mixed cultures? If one inoculates into an ultrapure 

 suspension of bacteria, which have acquired a refractory state, some 

 bacteriophage corpuscles of low virulence or even of moderate or high, 

 but not maximal, virulence these corpuscles are unable to parasitize 

 the bacteria and are eliminated after a few passages. At different times 

 I have even observed a complete destruction of the corpuscles by refrac- 

 tory bacteria.^2^ This fact has been confirmed by Flu.^^^ Indeed, a 

 destruction of the corpuscles must always occur when corpuscles having 

 a virulence below the maximum value are combined with susceptible 

 bacteria which have become refractory, for one may observe regularly, 

 after a few hours, a greater or smaller reduction in the number of 

 corpuscles. But it is only very infrequently that the destruction is 

 complete. 



If the resistance of the bacterium is limited and the virulence of the 

 bacteriophage is very high or maximum a fortiori bacteriophagy occurs 

 and the bacteria are destroyed. 



If the resistance of the bacteria is balanced by the virulence of the 

 corpuscles, and this is true whatever may he their respective values, a 

 symbiosis is estabhshed. We know that in a mixed culture each bac- 

 terium possesses its own more or less outspoken resistance, and in a 

 similar way each bacteriophage corpuscle possesses its own peculiar 

 virulence, more or less exalted. The more virulent corpuscles are able 

 to parasitize the bacteria of least resistance; but the bacteria of the 

 greatest resistance multiply even though the corpuscles have a very 

 high virulence. There occurs, then, under these conditions a continual 

 selection with the indefinite co-existence of the two antagonists. Such 

 mixed cultures, symbiotic in nature, can be subcultured indefinitely. 



* The respective behaviour of the bacterium and the bacteriophage is exactly 

 that of the seed of the orchid and of the fungus. Before the days of Noel Bernard 

 we were not familar with the processes of germination in the orchids, but through 

 his work we now know that fertilization takes place only when the seed is parasi- 

 tized by a fungus. The fungus penetrates into the seed and one of three results 

 may ensue. The resistance offered by the seed may overcome the virulence of the 

 parasite and the latter is then destroyed, or the virulence of the fungus outweighs 

 the resistance of the seed and here the seed is destroyed, or finally, the virulence 

 of the one and the resistance of the other balance, a symbiosis results, which in 

 effect is a limited parasitism. 



