Clonal spread of both oxyimino-cephalosporin- and cefoxitin-resistant Klebsiella pneumoniae isolates co-producing SHV-2a and DHA-1 β-lactamase at a burns intensive care unit

Over a 1-month period, a total of 16 ceftriaxone- and cefoxitin-resistant Klebsiella pneumoniae isolates were isolated from 15 patients hospitalised at a burns intensive care unit (ICU). These isolates showed negative results for extended-spectrum β-lactamase (ESBL) by the Vitek system and were highly resistant to ceftazidime, aztreonam and cefoxitin (minimum inhibitory concentrations ≥128 μg/mL). The blaSHV-2a and blaDHA-1 genes were detected by polymerase chain reaction and sequence analysis. Pulsed-field gel electrophoresis profiles of the isolates were identical. AmpC disk tests for AmpC enzymes as well as double-disk tests and Clinical and Laboratory Standards Institute (CLSI) confirmatory disk tests for ESBLs yielded positive results for all the isolates. However, only three isolates (18.8%) were shown to produce ESBL by CLSI confirmatory tests using broth microdilution. We report the first outbreak of colonisations and infections due to K. pneumoniae isolates co-producing an SHV-2a ESBL and a DHA-1 AmpC β-lactamase in a Korean hospital, which were suggested to represent a single clonal spread at a burns ICU. In addition, this report presents problems associated with ESBL detection using broth microdilution in isolates that co-produce an ESBL and an AmpC β-lactamase.