Concomitant conjugal transfer of reduced-bacteriophage-sensitivity mechanisms with lactose- and sucrose-fermenting ability in lactic streptococci.

Ten previously reported lactose-positive (Lac) transconjugants from Streptococcus lactis, S. cremoris, and S. lactis subsp. diacetylactis and one sucrose-positive (Suc) transconjugant from S. lactis were examined for their sensitivity to prolate- and small isometric-headed bacteriophages. Four of the Lac transconjugants showed a 10- to 100-fold reduction in the efficiency of plating (EOP) as well as a reduced plaque size for the prolate phage c2 and were insensitive to the small isometric phage 712. A fifth Lac transconjugant demonstrated a similar reduced sensitivity to phage c2; however, this transconjugant was able to plaque phage 712, but with a reduced plaque size and EOP. The other five Lac transconjugants were sensitive to both c2 and 712 phages. The Suc transconjugant plaqued phage 712 with a reduced plaque size and EOP, but no reduction in plaque size or EOP was observed for phage c2. The Lac and reduced bacteriophage sensitivity (Rbs) phenotypes were correlated with specific plasmids in the Lac transconjugants. As four of the Lac transconjugants exhibited a phenotypically indistinguishable Rbs, one (AB001) was selected for further study. The Rbs in AB001 for both small isometric- and prolate-headed phages was not related to adsorption, and the reduced EOP for phage c2 was not related to the presence of a restriction and modification system. The latent period for phage c2 was unchanged, but the burst size was reduced 80%. The presence of the plasmid coding for Rbs retarded the lysis of a mitomycin C-induced prophage-containing strain. The Rbs mechanism appears to be abortive phage infection. This study supports previous observations that Rbs and conjugal transfer ability are physically linked among some group N streptococci. The results presented have implications in the identification of plasmids coding for Rbs and may also aid in explaining the dissemination of Rbs genes among lactic streptococci.