Conjugal plasmid transfer between lactococci on solid surface matings and during cheese making

The concept of deliberate use of genetically enginereed microorganisms in dairy products requires a clear understanding of their behaviour and of the dissemination of introduced DNA in these strains. Thus, transfer of a self‐transmissible plasmid and a non‐self‐transmissible but mobilizable plasmid from an engineered strain of Lactococcus lactis subsp. lactis IL 1403 to wild‐type strains of L. lactis subsp. lactis and subsp. cremoris of technological interest was studied on standard solid surface matings and in cheese during manufacture. On solid surface matings, transfer of the conjugative plasmid occurred at frequencies ranging from < 2.3 × 10 −9 to 2.8 × 10 −4 . Mobilization of the non‐conjugative plasmid was observed at a lower frequency (ca. 10 −5 ) in only one recipient which was then selected along with another recipient strain (presenting intermediate transfer frequencies) for making Camembert cheese. During cheese making, only the transfer of the self‐transmissible plasmid was observed. It occurred in the early stages of manufacturing. The transfer frequencies were 7.0 × 10 −8 or 7.6 × 10 −1 1, depending upon the recipient strain. These were about 3 to 4 orders of magnitude lower than on solid surface matings. Mobilization of the non‐conjugative plasmid was never detected in cheese.