ISOLATION AND CHARACTERIZATION OF LARGE LACTOCOCCAL PHAGE RESISTANCE PLASMIDS BY PULSED-FIELD GEL-ELECTROPHORESIS

Five phage-resistant Lactococcus lactis strains were able to transfer by conjugation the lactose-fermenting ability (Lac(+)) to a plasmid-fr ee Lac(-) L. lactis strain. In each case, some Lac(+) transconjugants were phage-resistant and contained one or two additional plasmids of h igh molecular mass, as demonstrated by pulsed-field gel electrophoresi s. Plasmids pPF144 (144 kb), pPF107 (107 kb), pPF118 (118 kb), pPF72 ( 72 kb) and pPF66 (66 kb) were characterized: they are conjugative (Tra (+)), they confer a phage-resistant phenotype and they bear lactose-fe rmenting ability (Lactose plasmid) except for the last two. Plasmids p PF144, pPF107 and pPF118 resulted probably from a cointegrate formatio n between the Lactose plasmid and another plasmid of the donor strain, whereas pPF72, pPF66 and the Lactose plasmid were distinct in the cor responding transconjugants. Plasmids pPF72 and pPF66 produced a bacter iocin. At 30 degrees C, the phage resistance conferred by the plasmids was complete against small isometric-headed phage and partial against prolate-headed phage, except for pPF107 whose phage resistance mechan ism was totally effective against both types of phages, but was comple tely inactivated at 40 degrees C. Restriction maps of four of the plas mids were constructed using pulsed-field gel electrophoresis.