SUMMARY: The treatment of bacteria with antibiotics, chemotherapeutic agents or various chemicals influences the nature of the viruses with which the bacteria are infected. In the ease of many strains are infected with two or more distinct kinds of virus. Cultivation of these lysogenic strains in the presence of certain drugs may result in the removal of one of the viruses with a consequent alteration in the characters of the strain. Apart from changes in the host specificity of virus preparations yielded by the treated strains, these viruses may also differ in their capacity to transfer virulence to avirulent strains. For example, one diphtheria strain after treatment with certain antibiotics and chemicals developed the capacity to transfer virulence which it had not originally possessed. An avirulent strain after subcultivation in presence of certain drugs became susceptible to virulence-transfer from virus preparations not able to convey virulence to the parent strain. It appears that in lysogenic bacterial strains infected with two kinds of virus, one of the viruses may be more labile or easily removed than the other. Possibly the easily removable virus may be carried in the cytoplasm of the cell whilst the more stable virus may have some attachment to the nuclear apparatus of the bacterial cell. In one case investigated the stable virus was able to transfer virulence to an avirulent strain, whilst the cytoplasmic virus was not. It appears that there are two distinct mechanisms by which drugs may affect the viruses carried by bacterial strains: (i) by the selection of bacterial variants carrying a certain virus; (ii) by the preferential removal of the more labile viruses and inhibition of their multiplication. The results suggest criteria to be applied to the selection of potential chemotherapeutic agents for treating animal virus infections.


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