Lytic Plaque Formation and Variation in Virus Titre among Strains of Penicillium chrysogenum

Summary: Strains of Penicillium chrysogenum derived from a virus-infected culture have been investigated for reduction in virus titre. These strains were obtained following treatment with either heat (74°C) or specific antimetabolites. Virus titres were measured with a combination of immunological and radiochemical assays. One strain was cured of the mycophage and exhibited an increased growth rate and stability. Other strains, which show an apparent reduction in intracellular virus titre, actually released increased levels of virus from cells. Lytic plaques developed when infected strains were grown on an unbuffered, high lactose-containing medium, and virus titres could be correlated with plaque formation. All strains, regardless of virus titre, retained the ability to synthesize penicillin.