Discovery of CTX-M-Like Extended-Spectrum β-Lactamases in Escherichia coli Isolates from Five U.S. States

The most commonly reported extended-spectrum ␤-lactamases (ESBLs) in U.S. isolates of Escherichia coli are TEM and SHV derived (2).CTX-M ESBLs have been reported elsewhere, mostly for clinical isolates of Salmonella enterica serovar Typhimurium, E. coli, and Klebsiella pneumoniae, but not in the United States (5).In this report we describe nine U.S. isolates of E. coli from five different states (Virginia, Idaho, Ohio, Washington, and Texas) that appear to produce CTX-M-like ESBLs.These isolates were discovered as we began to investigate the types of ESBLs produced by organisms in a U.S. hospital surveillance study.All isolates were obtained from patient specimens during 2001-2002.The origins of the isolates, pIs of the ␤-lactamases, and NCCLS broth microdilution MICs (4) are shown in Table 1.Pulsed-field gel electrophoresis (PFGE) results after restriction with XbaI (7; R. V. Goering and F. C. Tenover, Letter, J. Clin.Microbiol.35:2432-2433, 1997) determined that all but isolates 1 and 4 were unique (data not shown).Isolates 8 and 9 were obtained from the same patient but had distinctly different antibiotic susceptibility and PFGE patterns.Cefepime MICs were generally higher than usual for TEM-and SHVderived ESBL-producing isolates, with cefepime MICs being Ն64 g/ml for seven of these isolates.Clavulanate-based ESBL tests confirmed ESBL production, although ceftazidime MICs for two isolates were Ͻ1 g/ml below the NCCLS ESBL screening criterion (4).Isoelectric focusing of crude sonicates of the isolates by a cefotaxime-␤-lactamase inhibitor overlay procedure (1, 6) revealed that all isolates produced a cefotaxime-hydrolyzing ␤-lactamase that was inhibited by clavulanate but not cloxacillin.The pIs of these enzymes were Ն9.0 for seven isolates and 8.0 for two (Table 1).All isolates except isolate 7 produced other clavulanate-inhibited ␤-lactamases.PCR amplification confirmed the presence of CTX-M-like genes within the isolates: isolates 1 to 7 resulted in amplified products of 414 bp