Assessment of extended-spectrum β-lactamase, KPC carbapenemase and porin resistance mechanisms in clinical samples of Klebsiella pneumoniae and Enterobacter spp.

The emergence and spread of resistance mechanisms in Gram-negative bacilli has complicated the treatment of serious nosocomial infections. Current automated systems for detection of Klebsiella pneumoniae carbapenemase (KPC)-producing isolates are unreliable. One possible straightforward alternative method is evaluation of ertapenem resistance. However, the accuracy of this method is affected by other resistance mechanisms such as AmpC gene expression or extended-spectrum β-lactamase production associated with porin loss. This study included 128 samples of K. pneumoniae and Enterobacter spp. that were non-susceptible to ertapenem. The disk diffusion and Etest method were applied to determine susceptibility to imipenem, meropenem and ertapenem. Isolates exhibiting intermediate or complete resistance to ertapenem were evaluated for resistance mechanisms. blaTEM, blaSHV, blaCTX-M, blaCTX-M-2 and blaKPC genes were tested for by PCR, and the presence of outer membrane protein was investigated by dot-blot assay. blaTEM was detected in 52.9% and 10.3%, blaSHV in 29.4% and 0.94%, blaCTX-M in 41.4% and 1.9% and blaCTX-M-2 in 23.5% and 1.9% of K. pneumoniae and Enterobacter cloacae isolates, respectively. The blaKPC gene was present in 12.6% of Enterobacter spp. isolates. OmpC and OmpF were present in 6.6% of E. cloacae isolates. These results indicate that several resistance mechanisms contribute to potential therapeutic failure of carbapenem therapy and point to the need for better detection methods and surveillance strategies.