Colonization of the digestive tract of germ-free mice by genetically engineered strains of Lactococcus lactis: study of recombinant DNA stability.

The ability of genetically engineered Lactococcus lactis strains to become established in the digestive tract (DT) of germ-free mice was examined together with the stability of their genetic markers. Seven L. lactis strains were genetically modified by insertion of genetic markers on different replicons: chloramphenicol resistance gene cat was carried by self-transmissible plasmid pIL205, a derivative of plasmid pIP501; erythromycin resistance gene erm, originating from pAM beta 1, was inserted into non-transmissible plasmids pIL252 and pIL253 of low and high copy number respectively; erm gene from plasmid pMS1.5B was inserted into the chromosome. All strains carried a common wild-type plasmid pIL9 involved in lactose fermentation. It was observed that the DT of mice was rapidly and efficiently colonized with either the inoculated parental strain or with its derivatives or with both of them, but plasmid-free derivatives were always at dominant levels. Both plasmids pIL9 and pIL205 were lost, but the parental strains and the plasmid-lacking derivatives were at codominant levels, indicating that there is an equilibrium between plasmid loss and plasmid transfer in the DT. Strains that carried non-transmissible and low copy number plasmid pIL252 were rapidly eliminated from the DT, which in turn was colonized with the respective pIL252-less derivatives; this is probably due to the high segregational instability of pIL252.(ABSTRACT TRUNCATED AT 250 WORDS)