Emergence of Klebsiella pneumoniae isolates producing inducible DHA-1 β-lactamase in a University Hospital in Taiwan

Ten nonrepetitive clinical isolates of Klebsiella pneumoniae exhibiting an unusual inducible β-lactam resistance phenotype were identified between January 1999 and September 2001 in a university hospital in Taiwan. In the presence of 2 μg of clavulanic acid, the isolates showed a one to four twofold concentration increase in the MICs of ceftazidime, cefotaxime, and aztreonam but remained susceptible to cefepime (MICs, ≤0.5 μg/ml) and imipenem (MICs, ≤0.5 μg/ml). PCR, sequence analysis, and isoelectric focusing revealed production by these isolates of TEM-1, SHV-11, and DHA-1, a plasmid-encoded inducible AmpC β-lactamase originally found in a Salmonella enterica serovar Enteritidis strain. Transfer of the resistance by conjugation experiments was not successful, but Southern hybridization showed that bla_DHA-1 was located on 70-kb plasmids, suggesting that the bla_DHA-1-containing plasmids in the K. pneumoniae isolates were non-self-transmissible. Five isolates were recovered from patients in two surgery wards and two intensive care units. Acquisition of the DHA-1 producers could be traced back to previous hospitalizations 1 to 5 months earlier for the other five patients. Six and seven patterns among the isolates were demonstrated by plasmid analysis and ribotyping, respectively, indicating that the spread of the DHA-1 producers was due to both horizontal transfer of bla_DHA-1 and dissemination of endemic clones.