HYPOTHESES CONCERNING NATURE OF BACTERIOPHAGE 319 



and develops at the expense of the bacterium, but this does not prevent 

 an indefinite continuance of the symbiosis. By isolation procedures 

 it is possible to obtain either pure bacteria, or a suspension containing 

 only the spores of the Myxomycete, but while the bacteria can be cul- 

 tivated in pure culture indefinitely, the spores of the Myxomycete 

 will not germinate and will not give birth to vegetative forms unless 

 the bacteria are present. 



I emphasize these examples of symbiosis in which, despite the parasi- 

 tism of the one of the two constituents, subcultures in series give symbio- 

 tic cultures indefinitely, for they show that, even if bacterial species 

 normally contained the bacteriophage principle, it could not be inter- 

 preted as absolute proof that the principle is derived from the bacte- 

 rium as such. It would still be necessary to exclude the chance that this 

 was not simply another example of the symbiotic relationship, a sym- 

 biosis between bacteriophage and bacterium, both beings being alive. 



The examples cited above offer a most perfect resemblance to the 

 situation existing when a bacterial strain is contaminated by bacterio- 

 phage corpuscles. As stated above it is quite possible to interrupt the 

 symbiosis between Zoochlorella and Paramecium or between Myxomy- 

 cete and Bacterium. And in the same way the symbiosis between 

 bacteriophage and bacterium can be interrupted, simply by applying 

 the usual bacteriological procedure for the purification of bacterial 

 cultures, that is, the method of colony isolation. After a series of two 

 or three transfers by means of isolated colonies, often even after the first, 

 the isolated colony will yield normal bacterial cultures no longer admixed 

 with the bacteriophage principle. 



Thus, we see, that, on the one hand, the implantation en bloc of a 

 bacterial culture containing the lysogenic principle gives an indefinite 

 series of cultures containing this principle, and that, on the other hand, 

 a simple seeding by means of isolated colonies is adequate to break this 

 symbiosis, for the re-seedings of isolated colonies give normal bacterial 

 cultures. There is an absolute parallelism between symbiosis involving 

 the Myxomycete and the Bacterium, and that involving the Bacterio- 

 phage and the Bacterium. The method of purification is the same in 

 both cases, and in both cases also, the normal bacterium can be further 

 cultivated alone in series, while the parasite, whether it be the Myxomy- 

 cete or the Bacteriophage, can be cultivated only in the presence of the 

 bacterium at whose expense it reproduces. 



It would require volumes to exhaust this question of symbiosis in 

 nature. The phenomenon is a general rather than an exceptional occur- 



