Chromosomal integration of the green fluorescent protein gene in lactic acid bacteria and the survival of marked strains in human gut simulations.

An integration vector was constructed to allow introduction of the gfp gene into the chromosomes of Gram-positive bacteria. Integration depends on homologous recombination between a short 458-nt sequence of the tet(M) gene in the vector and a copy of Tn916 in the host chromosome. Strains of Lactococcus lactis IL1403, Enterococcus faecalis JH2-SS, and Streptococcus gordonii DL1 stably marked with single chromosomal copies of the gfp were readily visualised by epifluorescence microscopy. The marked L. lactis strain survived poorly in a continuous culture system inoculated with human faecal flora, while the laboratory E. faecalis strain was lost at approximately the dilution rate of the fermenter.