SUMMARY: Mutants of phage W with diverse virulence were used to lysogenize naturally occurring strains of and their non-capsulogenic derivates. The temperate mutant β readily established itself as a prophage in both sporogenous and non-sporogenous strains of Strains carrying β-prophage were stable, capable of giving rise to colonies, and their multiplication was not impaired in the mouse. Thus, the virulence of capsulogenic strains, lysogenic for phage β, was not diminished. A less temperate mutant of phage W, phage α, was not capable of lysogenizing an atypical asporogenous strain of but formed more or less stable complexes with sporogenous strains. The stability of spores carrying phage genome α covered a wide range. Most of such spores did not give rise to colonies after germination but instead formed plaques. In some spores, however, phage α established itself as prophage so that stable lysogenic bacteria could be obtained under certain conditions. Spores of both capsulogenic and non-capsulogenic strains of lysogenic for phage α formed the same number of either plaques or colonies when plated on a sensitive indicator strain or spread on agar surface. When, however, the spores were spread on agar containing 0.025 M-sodium bicarbonate and the plates incubated at a high partial pressure of CO, then phage development in the lysogenic spores was induced. Hence, under these conditions, the lysogenic spores did not give rise to colonies but lysed and liberated phage. A small proportion of capsulogenic spores carrying prophage α threw off non-lysogenic segregants which gave rise to mucoid colonies in CO. These mucoid colonies consisted of encapsulated bacteria which probably remained non-lysogenic because their capsules prevented re-infection during their growth. Capsulogenic strains carrying prophage α were markedly less virulent in the mouse and this may be explained by induction of α-prophage by high CO tension in the animal body.

This competition between prophage induction and capsule formation, both of which were induced by high CO tension, explains the apparent diminution of the virulence of strains of lysogenic for α-phage.


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