Since genetically modified (GM) lactic acid bacteria (LAB) might be released in open
environments for future nutritional and medical applications, the purpose of this study
was to determine an upper limit for the horizontal gene transfer (HGT) in the digestive
tract (DT) from Lactococcus lactis carrying heterologous genes (lux genes encoding a
bacterial luciferase) to Enterococcus faecalis. Two enterococcal wide host-range
conjugative model systems were used: (i) a system composed of a mobilizable plasmid
containing the heterologous lux genes and a native conjugative helper plasmid; and (ii)
a Tn916-lux transposon. Both systems were tested under the most transfer-prone
conditions, i.e. germfree mice mono-associated with the recipient E. faecalis.
No transfer was observed with the transposon system. Transfers of the mobilizable plasmid
carrying heterologous genes were below 102 transconjugants per g of faeces for a single
donor dose and reached between 103 and 104 transconjugants per g of faeces when
continuous inoculation of the donor strain was used. Once established in mice,
transconjugants persisted at low levels in the mouse DT.