| Summary: | Objectives: To identify the mechanism of in vivo development of carbapenem resistance in Klebsiella pneumoniae.Methods: Seven sequential isolates of K. pneumoniae were obtained from twin infants with pneumonia. Antimicrobial susceptibility testing was performed by agar dilution. Carbapenemases including KPC and ML were initially screened using phenotypic methods, and carbapenemase-encoding genes were identified by PCR and amplicon sequencing. Plasmids of all clinical isolates and the conjugants of resistant isolates were estimated by S1 pulsed-field gel electrophoresis (PFGE). Molecular typing were conducted by PFGE of XbaI-digested genomic DNA and multilocus sequence typing (MLST). Results: For old brother, the first and third isolates were susceptible to meropenem, whereas the second and fourth isolates were resistant (MICs 16 mg/L). The first and second isolates from the young brother were susceptible to meropenem whereas the third isolate was resistant. All the resistant isolates produced NDM-1 metallo--lactamase. PFGE of XbaI-digested DNA revealed identical patterns for all the 7 isolates. All the isolates had the same sequence type named sequence type 37 (ST37). Conclusions: To our knowledge, this is the first documented case of development of carbapenem resistance in vivo mediated by NDM-1 metallo-b-lactamase in K. pneumoniae during treatment of pneumonia with meropenem.
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