SUMMARY: Further experimental evidence is described to support the concept that bacterial viruses exert a controlling effect on bacterial variation and evolution. A number of strains of diphtheria bacilli, both mitis and gravis, have been found to be carrying viruses capable of converting a susceptible avirulent diphtheria strain to full virulence and toxigenicity. The virus-resistant strains thus converted to virulence retain this property when subcultured and the change can be effected as well as The mechanism of virulence transfer has been investigated, and it appears that this virus-controlled conversion to virulence cannot be explained on the basis of the selection of virulent mutants already present in the avirulent culture. Also Iysogenicity in a diphtheria strain does not itself imply virulence, since some viruses which attack the avirulent strain give rise to resistant cultures which are lysogenic and carry the infecting virus but they remain avirulent and non-toxigenic. It remains possible that the infecting virus transfers genetic properties such as virulence from the original virulent host bacterium to the infected strain. Selection by the infecting virus is also involved, and when a bacterial strain becomes infected with a mixture of viruses one of these becomes dominant in any definite cultural condition, infects all the cells in the culture and thus produces a ‘homogenizing’ effect. When precautions are taken to limit this spread of virus throughout the bacterial population it is found that ordinary laboratory cultures of ‘pure’ bacterial strains contain variant bacteria with distinctive properties. These when isolated are found to be infected with mutant viruses. The origin of the virus affects its virulence-conveying property; for example, virulence has not yet been transferred by a virus originating from an avirulent strain. On the other hand, some viruses carried by virulent diphtheria strains have been found to be incapable of conveying virulence, although they are able to render the avirulent culture lysogenic. Not only the origin but also the subsequent history of a virus affects its virulence-conveying power. One virus preparation was found to retain its virulence-conveying ability indefinitely when propagated on the avirulent strain, whilst after one passage through a susceptible virulent strain the ability was lost.

Interpretations of the phenomena associated with lysogenicity are complicated by the frequent occurrence of bacteria simultaneously infected with several distinct viruses and by the possibility of hybridization effected by genetic recombination.


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