Emergence of Escherichia coli isolates producing conjugative plasmid-mediated DHA-1 β-lactamase in a Korean university hospital

Seven isolates of cefoxitin-resistant Escherichia coli with an inducible phenotype were detected between November 2002 and July 2003 in a Korean hospital. Conjugations were tested by the filter mating method using azide-resistant E. coli J53 as the recipient. All isolates and their transconjugants were tested for broth microdilution minimum inhibitory concentrations, isoelectric focusing (IEF), polymerase chain reaction (PCR) for SHV, TEM, CTX-M and DHA-derived β-lactamases, and DNA sequencing. XbaI-digested genomic DNA bands of the seven isolates were separated by pulsed-field gel electrophoresis (PFGE). IEF, PCR and sequence analysis revealed that all isolates possessed a bla_TEM-1-like and a bla_DHA-1 gene. Two isolates also carried the bla_CTX-M-14 gene. Transfer of the resistance by conjugation experiments of all seven isolates was successful, suggesting that the bla_DHA-1-containing plasmids in the E. coli isolates were self-transmissible. The isolates were recovered from patients in wards or an intensive care unit, all of which had been exposed to β-lactams before isolation of the DHA-1 producers. Five patterns among the seven isolates were demonstrated by PFGE; sporadic infections with E. coli possessing an inducible β-lactam resistance phenotype were found. DHA-1 encoded by conjugative plasmids conferred the resistance phenotype. The spread of the DHA-1 producers was due to both clonal spread and horizontal transfer of the resistance gene.