GENETIC-STUDY OF INDUSTRIAL LACTOCOCCUS-L ACTIS STRAINS - IDENTIFICATION OF TRANSMISSIBLE PLASMIDS WITH RESPECT TO THE LACTOSE FERMENTATIONTRAIT

Among 14 industrial strains of Lactococcus lactis, three strains - AC0 04, AC010, and AC013 - were found to be able to transfer the Lac(+) ph enotype to the plasmidless strain AC021, with a frequency of about 10( -7). The transconjugants originated from AC004 and AC013 strains and w ere capable bf subsequent transfer, with a frequency of similar to 10( -5), of the Lac(+) trait to the recipients AC018 and AC019, each conta ining two plasmids and sensitive to many bacteriophages. Transconjugan ts - derivatives of AC018 and AC019 - were also capable of subsequent transfer of the Lac(+) trait to the strain AC021 with a high (similar to 10(-2)) frequency. Phenotypic properties of the transconjugants wer e studied with respect to their ability to utilize lactose and galacto se and to their sensitivity to bacteriophages. Conjugative plasmids of about 53 and 87 kb, originating from AC004 and AC013, respectively, w ere found in cells of the Lac(+) transconjugants derived from AC018 an d AC019. A plasmid of similar to 87 kb resides in the original donor A C013, whereas a plasmid of similar to 53 kb seems to be formed via rec ombination. However, nether plasmid is detected in the transconjugants , derivatives of AC021, despite their ability to transfer the Lac(+) t rait. Meanwhile, in the AC021 recipient, as well as in AC019, a noncon jugative mobilizable similar to 55-kb plasmid, originating from AC010, was visualized. The data obtained suggest that conjugative plasmids i ntegrate into the chromosome of strain AC021.