SUMMARY: The adaptation of viruses to attack new hosts which originally are resistant greatly widens the sphere of influence of viruses on bacterial variation and evolution. The viruses attacking diphtheria bacilli are highly specific in their host range activity but many of them are readily adapted to lyse strains of other serological types. The range of adaptability of viruses is largely unknown and studies on the specificity and adaptability of bacterial viruses are complicated by the phenomenon of autoadaptability. Although virus-carrying bacterial strains are normally resistant to the viruses with which they are symbiotically infected it is now found that in many cases the viruses can be readily adapted to attack and lyse the parent bacterial cells. This has been found to occur spontaneously in cultures, and accounts for many cases of bacterial variation and dissociation. The existence of closed evolutionary chains in Iysogenic bacterial strains provides the explanation of many phenomena and difficulties in studies of phage typing and specificity. A further effect of significance in this field is the possibility of obtaining hybrid viruses by genetic recombination between an externally infecting virus and a virus already symbiotically infecting a bacterial cell. These effects can be elicited experimentally but they have also been observed to occur spontaneously in stock cultures and, it is suggested, may account for processes of bacterial variation and evolution controlled by viruses.


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