Mutation in a Bacterial Virus

Abstract

IN a recent paper¹, data have been given which suggest that certain of the symbiotic bacterial viruses (bacteriophages) found in Salmonella typhi-murium undergo diphasic variation. The normally innocuous symbionts occasionally throw off lytic mutants which multiply actively and destroy the host bacterium. This leads to the liberation of virus particles, some of which, when brought in contact with a susceptible host bacterium, develop as symbionts and reproduce the original type of infection, while others are lytic in nature and multiply actively at the expense of the bacterium. Of the particles generated in this lytic cycle the majority are themselves lytic; but a minority—perhaps about 5 per cent— are potential symbionts. In cultures on solid medium the latter produce symbiotically infected and hence ‘immune’ bacteria which multiply unscathed and form the characteristic ‘solid centre’ of the plaque.