The role of helper elements in the mobilisation of pBR recombinant plasmids (tra−, mob−, oriT+ and tra−, mob−, oriT−) from genetically engineered Escherichia coli K12 strains to other K12‐strains and to wild‐type E. coli strains of human faecal origin was examined. Transfer experiments were done in the digestive tract of axenic (germ free) and gnotobiotic mice, associated with human faecal flora, HFF. The kinetics of implantation of donors, recipients and transconjugants were determined. Mobilisation of oriT+ pBR‐type plasmids, by trans‐complementation with the products of tra and mob genes was obtained with E. coli K12, in the digestive tract of axenic mice and the resulting transconjugants became established together with the recipient and donor strains. Such mobilisation was only observed sporadically with one E. coli of human origin in axenic mice, but did not occur in gnotobiotic HFF mice. The E. coli strains of human origin were able to promote transfer of an oriT− pBR‐type plasmid in vitro but not in axenic or gnotobiotic mice. Transconjugants of wild‐type strains obtained in in vitro mating experiments and inoculated into gnotobiotic HFF mice were eliminated as rapidly as the recombinant K12 strains. This work indicates that ≥ 50% of wild‐type E. coli strains were able to promote transfer of pBR oriT− plasmids in vitro.

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Transfer of the shuttle vector pRRI207 mediated by the helper plasmid pRK2013 from Escherichia coli to Bacteroides spp. was possible in vitro and in vivo in the digestive tract of axenic mice associated with Bacteroides uniformis 1004 or Bacteroides vulgatus of human origin. In vivo, transfer frequencies were nearly identical for B. uniformis (2×10−7) and B. vulgatus (4×10−7) and the transconjugant strains of B. uniformis and B. vulgatus became established in the digestive tract of mice at densities ranging from 102–103 to 104 CFU g−1 of faeces, respectively. Transfer from E. coli to Bacteroides strains in gnotoxenic mice associated with human faecal flora (HFF) was not successful. Transconjugant‐like clones appeared among the HFF of gnotoxenic mice after they were inoculated with B. uniformis TBUA, a transconjugant strain of B. uniformis 1004 obtained from triparental mating and which harboured the shuttle vector. Hybridisation showed that the shuttle vector pRRI207 was not present in these clones, and it is suggested that they could result from the transfer of a conjugative transposon ERL contained in B. uniformis 1004. Moreover, clones believed to have lost the shuttle vector hybridised with a specific probe to B. thetaiotaomicron and therefore did not originate from B. uniformis TBUA.

We previously described the effects of intake of dairy products on plasmid dissemination in the digestive tract of gnotobiotic mice associated with human faecal flora (HFF) and found that yoghurt, heat-treated yoghurt (HTY) and milk reduced population levels of transconjugants compared with findings in mice fed a standard mouse diet. In the case of lactose intake, transconjugants were not detected. The aim of the present study was to assess the possible interrelationships between these observations and other variables (bacterial ecology, pH, moisture, enzyme activities, short-chain fatty acid (SCFA) contents, lactic acid contents). Much of the interest of the present comparison lies in the fact that the animals were homogeneous in terms of age, gender, food and intestinal microflora, owing to the gnotobiotic mouse model maintained in sterile isolators. We observed no variation in SCFA and lactic acid contents or in the population levels of strictly anaerobic strains of Bacteroides and Bifidobacterium, and of the facultative anaerobic recipient Escherichia coli PG1 strain. The main modifications were the reduction of population levels of transconjugants in mice receiving yoghurt, HTY and milk, and concomitantly an increase of b-galactosidase and a decrease of b-glucosidase activities, compared with control mice fed a standard diet. Total inhibition of plasmid transfer was observed in HFF mice consuming lactose, and concomitantly the two enzyme activities (b-glucosidase and b-galactosidase) were increased, compared with the findings in control mice fed a standard diet. In axenic mice consuming lactose, plasmid transfer occurred, b-galactosidase was not detected and b-glucosidase was decreased. It is therefore proposed that these two enzyme activities influence plasmid transfer and persistence of transconjugants in the digestive tract of HFF associated mice. When both activities were increased there was a total inhibition of plasmid transfer (case of lactose intake). When b-galactosidase increased and b-glucosidase decreased (case of yoghurt, HTY and milk), plasmid transfer occurred at a lower efficiency than in the control group, resulting in lower population levels of transconjugants.

This study deals with the effects of yoghurt intake on wild-type and recombinant plasmid transfer from an exogenous Escherichia coli K12-derivative donor strain to an endogenous recipient strain in the digestive tract of mice associated with human faecal flora. We showed that the self-transmissible plasmid R388 was efficiently transferred to recipient strain PG1 in mice associated with human faecal flora (HFF-PG1) and that the resulting transconjugants (PG1-R388) became established at a high and maximal population level without any selective pressure. Using HFF-PG1 mice made it possible to determine whether yoghurt consumption decreases R388 transfer efficiency and the ability of transconjugant PG1-R388 to successfully colonise the digestive tract. Results indicated that yoghurt consumption had two effects: it reduced the efficacy of plasmid transfer 10-fold and decreased the transconjugant PG1-R388 population density 100-fold, compared to the control group. We also describe another HFF mouse model in which recipient PG1 was replaced by EM0 with which no plasmid transfer was observed. This model made it possible to demonstrate the potential promoting effect of yoghurt intake on transconjugant formation and establishment. Our results indicated no yoghurt effect; no transconjugants appeared in the digestive tract of HFF-EM0 mice fed on yoghurt or on standard food. In both mouse models, HFF-PG1 and HFF-EM0, yoghurt intake did not promote the mobilisation of either the non-self-transmissible plasmid pUB2380 or the recombinant plasmid pCE325.