Transfer of Plasmid pAMβl Between Members of the Normal Microflora Inhabiting the Murine Digestive Tract and Modification of the Plasmid in a Lactobacillus reuteri Host

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Abstract
The fate of the broad-host-range, conjugative Plasmid pAMβl in the digestive tract of mice was monitored by DNA DNA hybridisations using a probe containing the ermAM determinant (macrolide-lincosamide-streptogramin type B resistance). Transfer of pAMβl from Lactobacillus reuteri to Enterococcus faecium occurred in the digestive tract of infant mice and from E. faecium to E. faecalis and to L. fermentum in the adult intestinal tract. Long-term administration of lincomycin to the animals resulted in a modified form of pAMβl in the L. reuteri host. The modified plasmid was present in caecal isolates of the lactobacillus at higher concentration than in the stock culture used to inoculate the mice at the start of the experiment, and was maintained stably by the lactobacilli in vivo. In contrast, pAMβl was not stably maintained in the stock strain in the absence of antibiotic administration. The modified plasmid was no longer self-transmissible. Long-term administration of antibiotic to the animals resulted in erythromycin-resistant strains of Escherichia coli and eubacteria.