In vivo transmission of a plasmid coharbouring bla<inf>DHA-1</inf> and qnrB genes between Escherichia coli and Serratia marcescens

We report a Serratia marcescens and an Escherichia coli isolate simultaneously detected in the same patient. Both isolates showed susceptibility patterns suggestive of harbouring a plasmid-mediated AmpC β-lactamase (pACBL) and a plasmid-encoded quinolone resistance (PMQR). PCR-based replicon, MOB typing, plasmid profile and Southern hybridization analyses revealed that both isolates coharboured blaDHA-1 and qnrB genes on the same IncL/M-MOBP13 plasmid approximately 70 kb in size. Together with the fact that both plasmids were conjugative in the laboratory, these results strongly suggest that a horizontal transfer event could take place in vivo. This is the first report of an isolate of S. marcescens harbouring a pACBL. The only phenotypic method that suggests the presence of a pACBL in an isolate harbouring an inducible chromosomal AmpC enzyme is the observation of scattered colonies near the edge of the inhibition zones of some β-lactams. The presence of both resistance genes on the same plasmid and the reported increase in PMQR could perhaps explain the widespread distribution of blaDHA-1 genes. © 2010 Federation of European Microbiological Societies. Published by Blackwell Publishing Ltd.