Plasmid-mediated, inducible AmpC beta-lactamase (DHA-1)-producing Enterobacteriaceae at a Korean hospital: wide dissemination in Klebsiella pneumoniae and Klebsiella oxytoca and emergence in Proteus mirabilis.

The aim of the study was to investigate the phenotypic and genetic characteristics of recently emerging cefoxitin-resistant and induction-positive isolates of Escherichia coli, Klebsiella species, and Proteus mirabilis. Strains of Enterobacteriaceae were isolated at a Korean tertiary care hospital between June and December 2002. Induction was tested using cefoxitin and aztreonam disks, the blaDHA allele was detected by PCR, and pulsed-field gel electrophoresis (PFGE) patterns were also analyzed. Among the cefoxitin-resistant isolates, 2.7% of E. coli, 21.1% of Klebsiella pneumoniae, 32.0% of Klebsiella oxytoca, and 8.3% of P. mirabilis isolates showed induction, and were blaDHA-1 allele positive. To the best of our knowledge, this is the first report of blaDHA-1 in P. mirabilis. The MICs of ceftazidime, cefotaxime, and aztreonam increased significantly by higher inoculum, suggesting that their clinical usefulness is limited. Presence of multiple PFGE patterns and identical patterns in some isolates suggest that the widely disseminated blaDHA-1 in Klebsiella species was because of both horizontal and clonal spread.