Mechanism of plasmid-mediated bacteriophage resistance in lactococci

Abstract Plasmids pBu1-B and p1149-3 from Lactococcus lactis ssp. lactis biovar. diacetylactis Bu1 and L. lactis ssp. cremoris 1149 mediate resistance against a variety of mostly isodiametrically headed bacteriophages. Plasmid pBu1-8 was conjugally transferred into several plasmid-free, phage sensitive strains of both lactococcal subspecies rendering all transconjugants phage resistant. Infection with bacteriophages P008 and P416 labeled in vivo with the fluorescence dye 4'-6-diamidino-2-phenylindole (DAPI) showed that adsorption and DNA injection took place with both sensitive and resistant strains even after extensive starvation of the host cells. In resistant cells carrying either of the plasmids, phage DNA was replicated over more than 5 h post-infection. No transcription of the phage DNA, however, could be observed.