A novel ceftazidime-hydrolysing extended-spectrum beta-lactamase, CTX-M-54, with a single amino acid substitution at position 167 in the omega loop.
OBJECTIVE
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 beta-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 approximately 21 kbp that carries both blaSHV-2a and blaCTX-M-54 beta-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 beta-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.
Bae
Il Kwon
IK
Department of Laboratory Medicine, Kosin University College of Medicine 602-030, 34 Amnam-Dong, Suh-Gu, Busan, Korea.
Lee
Byung Ho
BH
Hwang
Hyun Yong
HY
Jeong
Seok Hoon
SH
Hong
Seong Geun
SG
Chang
Chulhun L
CL
Kwak
Hyo-Sun
HS
Kim
Hyoung Jin
HJ
Youn
Hasik
H
D019943:Amino Acid Substitution; D000900:Anti-Bacterial Agents / Q000494:pharmacology; D001483:Base Sequence; D002442:Ceftazidime / Q000378:metabolism* / Q000494:pharmacology; D004251:DNA Transposable Elements / Q000235:genetics; D004269:DNA, Bacterial / Q000235:genetics; D024881:Drug Resistance, Bacterial / Q000235:genetics*; D005260:Female; D006801:Humans; D007710:Klebsiella Infections / Q000382:microbiology; D007711:Klebsiella pneumoniae / Q000187:drug effects / Q000201:enzymology*; D008826:Microbial Sensitivity Tests; D008969:Molecular Sequence Data; D010957:Plasmids / Q000235:genetics; D016133:Polymerase Chain Reaction; D017422:Sequence Analysis, DNA; D001618:beta-Lactamases / Q000235:genetics* / Q000302:isolation & purification / Q000378:metabolism*; D047090:beta-Lactams / Q000378:metabolism / Q000494:pharmacology
D000900:Anti-Bacterial Agents; D004251:DNA Transposable Elements; D004269:DNA, Bacterial; D047090:beta-Lactams; D002442:Ceftazidime; D001618:beta-Lactamases