JAC Advance Access originally published online on June 17, 2006
Journal of Antimicrobial Chemotherapy 2006 58(2):315-319; doi:10.1093/jac/dkl252
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A novel ceftazidime-hydrolysing extended-spectrum ß-lactamase, CTX-M-54, with a single amino acid substitution at position 167 in the omega loop
1 Department of Laboratory Medicine, Kosin University College of Medicine 602-030, 34 Amnam-Dong, Suh-Gu, Busan, Korea 2 Department of Laboratory Medicine, Pochon CHA University College of Medicine 463-712, 351 Yatap-Dong, Bundang-Gu, Sungnam, Korea 3 Department of Laboratory Medicine, Pusan National University School of Medicine 602-739, Suh-Gu, Ami-Dong 1-10, Busan, Korea 4 Center for Food Safety Evaluation, Korea Food and Drug Administration 122-704, 231 Jinheung-Ro, Eunpyung-Gu, Seoul, Korea 5 R&D Park, LG Life Sciences Ltd 305-380, 104-1 Moonji-Dong, Yuseong-Gu, Daejeon, Korea
Received 25 April 2006; returned 16 May 2006; revised 19 May 2006; accepted 24 May 2006
*Corresponding author. Tel: +82-51-990-6373; Fax: +82-51-990-3034; E-mail: kscpjsh{at}ns.kosinmed.or.kr
Objectives: To characterize a novel ceftazidime-hydrolysing CTX-M mutant, designated CTX-M-54, produced by Klebsiella pneumoniae clinical isolate BDK0419 and to investigate its genetic environment.
Methods: Antimicrobial susceptibilities were determined by disc diffusion and agar dilution methods, and the double-disc synergy test was carried out. Detection of genes encoding class A ß-lactamases was performed by PCR amplification, and the genetic organization of the blaCTX-M-54 gene was investigated by PCR and sequencing of the regions surrounding this gene. Kinetic parameters were determined from purified CTX-M-54.
Results: The strain BDK0419 contained a transferable plasmid with a molecular size of 
21 kbp that carries both blaSHV-2a and bla CTX-M-54 ß-lactamase genes, along with two other plasmids. The blaCTX-M-54 gene was flanked upstream by an ISEcp1 insertion sequence and downstream by an IS903-like element. CTX-M-54 had a P167Q substitution within the omega loop region of class A ß-lactamases compared with the sequence of CTX-M-3. The MIC of ceftazidime for K. pneumoniae BDK0419 was 16-fold higher than that of cefotaxime; however, the kinetic parameter of CTX-M-54 against ceftazidime revealed a low catalytic efficiency.
Conclusions: This work shows once again that novel CTX-M enzymes with an expanded activity towards ceftazidime through a single amino acid substitution can be identified from clinical isolates. Thus, detection of CTX-M enzymes can no longer be based solely on the resistance phenotypes of clinical isolates towards ceftazidime and cefotaxime.
Keywords: cefotaximase , ISEcp1 , IS903
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