Interactions of temperate bacteriophages ofStreptococcus salivariussubsp.thermophiluswith lysogenic indicators affect phage DNA restriction patterns and host ranges

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Abstract
After treatment with mitomycin C, 12 of the 120 strains of the French Collection of Lactic Acid Bacteria (CNRZ) belonging to the species Streptococcus salivarius subsp. thermophilus were found to be inducible and lysogenic. The corresponding temperate phages were multiplied on indicator strains and further characterized. It is noteworthy that some of the indicators were themselves lysogenic. The temperate phages belonged to Bradley's group B, or to the Siphoviridae family of the International Committee on Taxonomy of Viruses. Seven of them were shared among the two protein profiles previously established for virulent phages of Str. salivarius subsp. thermophilus. Seven different DNA restriction profiles were found for ten native temperate phages examined just after mitomycin C induction. Genome sizes varied from 40 to 45 kb and were classified into four related homology groups by DNA cross-hybridization, but there was no clear-cut relationship with the protein clusters previously shown. DNA homology with representatives of virulent phages was also found. The DNA restriction profiles of seven native temperate phages, examined just after mitomycin C induction of the lysogenic hosts, were noticeably different from those of the corresponding phages once they had been propagated on lysogenic indicators. Moreover, the host range of the latter phages was extended to a greater number of sensitive strains. The possible role of lysogenic strains of Str. salivarius subsp. thermophilus in the occurrence of phage outbreaks affecting this dairy streptococcal species is discussed.