Prevalence and resistance patterns of extended-spectrum and AmpC β-lactamase in Escherichia coli, Klebsiella pneumoniae, Proteus mirabilis, and Salmonella serovar Stanley in a Korean tertiary hospital.

A total of 100 clinical isolates of Escherichia coli (n = 35), Klebsiella pneumoniae (n = 63), Proteus mirabilis (n = 1), and Salmonella serovar Stanley (n = 1), showing resistance to cefoxitin, or returning positive in extended-spectrum β-lactamase (ESBL) by Clinical and Laboratory Standards Institute (CLSI) ESBL confirmatory method, were studied. The isolates were examined by the boronic acid (BA) disk test, polymerase chain reaction, and pulsed-field gel electrophoresis (PFGE) to investigate genetic similarities. The concurrence rates for ESBLs by the CLSI and the BA disk test were 97% for E. coli and 96.7% for K. pneumoniae. A total of 41 isolates showing cefoxitin resistance yielded all positive by the BA disk test. All the 33 K. pneumoniae isolates, which showed positive by the BA disk test, were carrying AmpC genes. The TEM and CTX-M types were predominant in E. coli and the SHV and the CIT and/or DHA types were predominant in K. pneumoniae. PFGE analysis showed almost 75% of genetic similarities among K. pneumoniae isolates producing ESBLs and/or AmpC β-lactamases (AmpCs) as each K. pneumoniae carried variable genes and showed variable antibiotic patterns. Clearly, the BA disk test was a useful method for the detection of ESBLs and AmpCs. In particular, cefoxitin resistance and BA-positive trait of K. pneumoniae do reflect the presence of AmpC genes in the organism.