76 THE BACTERIOPHAGE 



culture in 1. The second flask is portioned out the next day, that is, after 

 lysis is completed, 10 cc. being placed in each of 20 tubes. None of these 

 become turbid. When this second part of the experiment is repeated, 18 

 remain clear, and 2 tubes yield secondary cultures. 



Each flask of suspension contained 50,000 million bacilli, and 

 the above experiments show that of this number but one or two 

 were capable of acquiring an immunity to the very active bacterio- 

 phage. It is these "immune" bacilli which give rise to organisms 

 that enjoy the same degree of resistance. 



Secondary cultures, then, have their origin in the operation of 

 the phenomenon of natural selection, whereby some bacilli show 

 a greater aptitude than others to the acquisition of a resistance to 

 the bacteriophage. 



The phenomenon of secondary culture formation is governed 

 by the individual properties of the two admixed organisms, 

 bacterium and bacteriophage. Against a single strain of bac- 

 terium the less virulent the bacteriophage the greater will be the 

 proportion of secondary cultures, or, in other words, the greater 

 is the number of bacilli in the suspension capable of acquiring a 

 resistance. 



Against a given strain of bacteriophage the different strains of 

 a single bacterial species are not endowed with an equal resistance. 

 With certain strains secondary cultures will be the rule, with 

 others, the exception, and with still others, they will never occur. 



We will shortly see the reasons for this variation; at present we 

 may say that the degree of resistance possessed by a bacterium 

 to a bacteriophage is, for a given bacterial species, in direct 

 relation to the degree of virulence which this bacterial strain 

 possesses for the higher organism which it is capable of invading. 



INSTABILITY OF MIXED CULTURES 



Mixed cultures result from a state of equilibrium between the 

 virulence of the bacteriophage and the resistance of the bacterium. 

 But these two factors are by nature variable and vary in intensity 

 from one time to another, being influenced by the circumstances 

 of the moment. This equilibrium can be interrupted experi- 

 mentally in either direction, so as to favor either the one or the 

 other of the factors. 



