Molecular analysis of a prolonged spread of Klebsiella pneumoniae co-producing DHA-1 and SHV-12 β-lactamases.

The study investigated molecular mechanisms for prolonged nosocomial spread of multidrug-resistant Klebsiella pneumoniae co-producing plasmid-mediated AmpC β-lactamase DHA-1 and extended-spectrum β-lactamase SHV-12. Forty-eight clinical isolates of K. pneumonia, resistant to the extended-spectrum cepha-losporins, were collected in a 750-bed university hospital over a year. The isolates were characterized for PCR-based β-lactamase genotypes, isoelectric focusing and pulsed-field gel electrophoresis (PFGE) profiles. Resistance transfer was performed by plasmid conjugation and confirmed by a duplex-PCR and Southern hybridization. On β-lactamase typing, the strains producing only the DHA-1 enzyme (n=17) or co-producing DHA-1 and SHV-12 enzymes (n=15) were predominant. Judging from a one year-distribution of PFGE profiles, the co-producer was spread primarily with single clonal expansion of the PFGE-type A with subtypes (n=14), whereas the strains producing only DHA-1 enzyme were spread simultaneously with the PFGE-type A (n=ll) and other PFGE types (n=6). Transconjugants of the co-producers were confirmed to harbor either both bla (DHA-1) and bla (SHV-12) or only the bla (DHA-1). In conclusion, this study indicated that the persistent nosocomial spread of multidrug-resistant K. pneumoniae strains was primarily associated with expansion of a clone harboring both the bla (DHA-1) and bla (SHV-12) or the bla (DHA-1) only, and to a lesser extent with the horizontal transfer of the resistant plasmids. Our observations have clinical implication for the control and prevention of nosocomial dissemination of multidrug-resistant K. pneumoniae strains.