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

Abstract

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.