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Abstract
We studied the incidence of inducible Lactobacillus helveticus strains among 74 isolates from natural whey starter cultures that are used for the production of Italian Grana and Provolone cheeses. Mitomycin C was used for induction of cultures. Of the 77 strains tested, 35 strains (47%) released supernatant particles in the culture that were able to inhibit up to a maximum of 47 (61%) sensitive indicator strains. The presence of bacteriophages in culture supernatants of L. helveticus strains LH38 and LH60 was verified by electron microscopy. None of the positive culture supernatants formed plaques, and inhibition zones were observed only on agar plates that had been inoculated with the indicator strains. Fourteen of the 17 positive culture supernatants were able to propagate on some indicator strains, indicating the presence of completely infective virions in liquid medium. The particles were insensitive to proteolytic enzymes, were unable to pass through a microfiltration membrane, were sensitive to heat, and were sedimented by ultracentrifugation. Some of these particles showed killer activity against nonproliferating cells of indicator strains. The lysogenic state of the L. helveticus strains was related to a stronger resistance of these strains to the products of the induction; higher prevalence of indicator strains were found among the noninducible strains.
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