Genetic Characterization of Multiple Mechanisms of Phage Defense from a Prototype Phage-Insensitive Strain, Lactococcus lactis ME2

Abstract

Lactococci used as starter cultures in dairy fermentations are highly susceptible to attack by bacteriophage. Genetic studies with Lactococcus lactis ME2, a prototype phage-insensitive strain, have identified plasmid-encoded defenses, which interfere with phage adsorption, restrict and modify phages, or abort lytic phage infection. Restriction and modification and abortion of phage infection were localized on two distinct self-transmissible plasmids, pTN20 and pTR2030, respectively, originating from L. lactis ME2. A comparison of the physical and genetic characteristics of these two conjugative plasmids is presented. Conjugation and cloning strategies employed to assemble these complementary mechanisms of phage resistance will be discussed. The collective expression of different defense systems provided a greater phage resistance to dairy lactococci. Starter cultures that are recalcitrant to phage attack can be constructed from existing strains through application of genetic technologies, which assemble complementary mechanisms of phage defense.