Indications of in vivo transfer of an epidemic R plasmid from Salmonella enteritidis to Escherichia coli of the normal human gut flora

The presence of conjugative R plasmids as well as the possible similarities among them were studied in nine ampicillin-resistant Salmonella enteritidis isolates and nine ampicillin-resistant Escherichia coli isolates from the normal fecal flora that were simultaneously isolated from nine epidemiologically unrelated outpatients. It was found that in eight patients, ampicillin resistance in S. enteritidis was encoded by ca. 34-MDa transferable plasmids very similar to those found in a recent study of the epidemiology of ampicillin-resistant S. enteritidis in Greece (A. C. Vatopoulos, E. Mainas, E. Balls, E. J. Threlfall, M. Kanelopoulou, V. Kalapothaki, H. Malamou-Lada, and N. J. Legakis, J. Clin. Microbiol. 32:1322-1325, 1994). Moreover, transferable R plasmids with the same molecular size and restriction pattern were found in the normal flora E. coli of two of these patients. This finding, if confirmed by further studies, is consistent with the hypothesis that normal flora E. coli could act as a reservoir of resistant genes and, consequently, as a factor in the dissemination of these genes among pathogens of human and animal origin such as Salmonella spp. and needs to be examined further.