Evidence for temperate bacteriophages in two strains of Lactobacillus bulgaricus

Lactobacillus bulgaricus strains LT1 and LT4 previously isolated from a yogurt factory were shown to be lysogenic and inducible by mitomycin C or u.v. irradiation, induction being optimal at the beginning of exponential growth. Five indicator strains (four of L. lactis, one of L. bulgaricus) were found in which the two phage lysates propagated well in a liquid medium, but formed plaques on bacterial lawns less readily. Ca2+ was required for the phage infection of the indicator strains to reach termination. Phage particles from each strain exhibited a comparable ultrastructural morphology under the electron microscope, having an isometric head, a triple collar and a noncontractile fibre-containing tail.