A novel extended-spectrum beta-lactamase CTX-M-23 with a P167T substitution in the active-site omega loop associated with ceftazidime resistance.
OBJECTIVE
In recent years, cefotaximases of the CTX-M type have become a predominant cause of resistance to extended-spectrum cephalosporins in Gram-negative bacteria. Although most enzymes provide higher levels of resistance to cefotaxime than to ceftazidime, mutants with enhanced catalytic efficiency against ceftazidime have recently been described. This report identifies another ceftazidime-resistant mutant of the CTX-M class of enzymes.


METHODS
Two ceftazidime-resistant strains, Escherichia coli IFI-1 and Klebsiella pneumoniae IFI-2, were isolated from a 46-year-old man during treatment of postoperative peritonitis with ceftazidime. Susceptibility testing, mating-out assays, isoelectric focusing as well as PCR and sequencing techniques were carried out to investigate the underlying mechanism of resistance.


RESULTS
E. coli IFI-1 and K. pneumoniae IFI-2 exhibited a clavulanic acid-inhibited substrate profile that included extended-spectrum cephalosporins. Notably, both strains had up to a 32-fold higher level of resistance to ceftazidime than to cefotaxime. Further characterization revealed that a novel bla(CTX-M) gene encoding a beta-lactamase with a pI of 8.9 was implicated in this resistance: CTX-M-23. Along with the substitutions D114N and S140A, CTX-M-23 differed from CTX-M-1, the most closely related enzyme, by a P167T replacement in the active-site omega loop, which has not previously been observed in other CTX-M enzymes. By analogy with what was observed with certain TEM/PSE/BPS-type beta-lactamases, the amino acid substitution in the omega loop may explain ceftazidime resistance, which has only rarely been reported for other CTX-M enzymes.


CONCLUSIONS
The emergence of a new ceftazidime-resistant CTX-M-type mutant provides evidence that these enzymes are able to broaden their substrate spectrum towards ceftazidime, probably due to substitutions in the active-site omega loop.
Stürenburg
Enno
E
Institut für Infektionsmedizin, Zentrum für Klinisch-Theoretische Medizin I, Universitätsklinikum Hamburg-Eppendorf, Martinistrasse 52, D-20246 Hamburg, Germany. e.stuerenburg@uke.uni-hamburg.de
Kühn
Alexandra
A
Mack
Dietrich
D
Laufs
Rainer
R
D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D002442:Ceftazidime / Q000494:pharmacology*; D018550:Cephalosporin Resistance / Q000235:genetics*; D002511:Cephalosporins / Q000494:pharmacology*; D017931:DNA Primers; D004926:Escherichia coli / Q000187:drug effects / Q000201:enzymology / Q000235:genetics; D029968:Escherichia coli Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism*; D006801:Humans; D007525:Isoelectric Focusing; D007711:Klebsiella pneumoniae / Q000187:drug effects / Q000201:enzymology / Q000235:genetics; D008297:Male; D008875:Middle Aged; D008969:Molecular Sequence Data; D011183:Postoperative Complications / Q000382:microbiology; D020133:Reverse Transcriptase Polymerase Chain Reaction; D065093:beta-Lactamase Inhibitors; D001618:beta-Lactamases / Q000378:metabolism*
D002511:Cephalosporins; D017931:DNA Primers; D029968:Escherichia coli Proteins; D065093:beta-Lactamase Inhibitors; D002442:Ceftazidime; D001618:beta-Lactamases; C492230:beta-lactamase CTM-X-23, Klebsiella pneumoniae; C492229:beta-lactamase CTX-M-23, E coli