Abstract
Recently, carbapenem-resistant Gram-negative bacteria belonging to the family Enterobacteriaceae (CRE) have become a general clinical concern, but it is very difficult to elucidate the molecular mechanisms underlying their carbapenem resistant phenotype. In this study, three Escherichia coli strains (NUBL-5310, NUBL-5317, and NUBL-9600), two Klebsiella pneumoniae strains (NUBL-5307, and NUBL-5309), one Enterobacter aerogenes strain (NUBL-5311), and one Enterobacter cloacae strain (NUBL-7700) demonstrating carbapenem nonsusceptibility or cephem resistant phenotypes were subjected to an antimicrobial susceptibility test, β-lactamase typing using β-lactamase inhibitors, and polymerase chain reaction (PCR). Conjugal transfer of plasmid, transformation, gene cloning, and nucleotide sequence analyses were additionally performed in some isolates. In E. cloacae NUBL-7700, genes for blaIMP and blaCTX-M-1 group were detected by PCR. In the remaining six strains exhibiting reduced susceptibility or resistance profiles to carbapenems, no new carbapenemase was detected, but genes for CMY-, DHA-, or CTX-M-type β-lactamases were detected in six isolates. Increased production of chromosomal AmpC was speculated in E. aerogenes NUBL-5311 because aminophenyl boronic acid reduced the resistance to cephems and no genes for plasmid-mediated β-lactamases were detected by PCR. Augmented production of CMY-, DHA-, or CTX-M-type β-lactamases and the decrease or deletion of certain outer membrane proteins probably contribute to the carbapenem-nonsusceptible phenotypes among the isolates tested.
