Characterization of Phage-Sensitive Mutants from a Phage-Insensitive Strain of Streptococcus lactis: Evidence for a Plasmid Determinant that Prevents Phage Adsorption

A phage-insensitive strain of Streptococcus lactis, designated ME2, was used as a prototype strain for the study of mechanisms and genetics of phage resistance in the lactic streptococci. Mutants sensitive to a Streptococcus cremoris phage, varphi18, were isolated at a level of 17% from cultures of ME2 after sequential transfer at 30 degrees C. Phage-sensitive mutants of ME2 were not fully permissive to varphi18. The efficiency of plating of varphi18 on the mutants was 5 x 10 as compared with <10 for varphi18 on ME2. Further characterization of the mutants showed that they efficiently adsorbed varphi18 at levels of >99.8%, whereas ME2 adsorbed only 20 to 40% of varphi18. These results suggest that increased phage susceptibility of the mutants may result from the loss of a mechanism that inhibits phage adsorption. Moreover, the high frequency of spontaneous mutation in ME2 indicates the involvement of an unstable genetic determinant in this phage defense mechanism. ME2 was shown to possess 13 plasmids ranging in size from 1.6 to 34 megadaltons. Of 40 mutants examined that had increased efficiencies of plating, all were missing a 30-megadalton plasmid, pME0030. These data suggest that pME0030 codes for a function that prevents phage adsorption. Further phenotypic characterization of the phage-sensitive mutants showed that some mutants were deficient in the ability to ferment lactose (Lac) and hydrolyze milk proteins (Prt). However, the Lac and Prt phenotype segregated independently of the phage-sensitivity phenotype. One phage-sensitive adsorption mutant, designated N1, was tested for susceptibility to 14 different phages. N1 showed increased capacity to adsorb 4 and to replicate 2 of these 14 phages, thereby indicating a phage resistance mechanism in ME2 that generalizes to phage interactions other than the specific varphi18-ME2 phage-host interaction. These data provide evidence for a unique plasmid-linked phage defense mechanism in phage-insensitive strains of lactic streptococci.