In 271 Enterobacter blood culture isolates from 12 hospitals, extended-spectrum beta-lactamase (ESBL) prevalence varied between 0% and 30% per hospital. High prevalence was associated with dissemination, indicating the potential relevance of infection control measures. Screening with cefepime or Vitek 2, followed by a cefepime/cefepime-clavulanate Etest, was an accurate strategy for ESBL detection in Enterobacter isolates (positive predictive value, 100%; negative predictive value, 99%).No guidelines have been issued for extended-spectrum betalactamase (ESBL) detection in members of the Enterobacteriaceae with inducible chromosomal AmpC beta-lactamases, such as Enterobacter spp., although these species may cause hospital outbreaks (7, 9, 10), are frequently multidrug resistant (MDR) (9, 10), and may constitute a reservoir for plasmidmediated ESBLs (3, 12, 13) for other Enterobacteriaceae species. The absence of such recommendations likely has two reasons. First, phenotypic detection of ESBLs in members of the Enterobacteriaceae coexpressing an AmpC beta-lactamase is complex, because AmpC expression may mask the synergy required for ESBL detection between third-generation cephalosporins and clavulanic acid. This problem may be circumvented by demonstrating synergy between clavulanic acid and cefepime, a fourth-generation cephalosporin hydrolyzed by ESBLs but generally not by AmpC beta-lactamases (11, 13). Second, the outcome of an ESBL detection test in Enterobacteriaceae spp. with inducible AmpC is considered of limited therapeutic consequence, since most clinicians consider cephalosporins, including cefepime, inappropriate for treatment of infections caused by these species (6, 8). As a result, data are sparse on ESBL prevalence in Enterobacteriaceae spp. with inducible chromosomal AmpC beta-lactamases.The aims of this study were (i) to develop a phenotypic ESBL detection strategy in Enterobacter species for the routine clinical laboratory and (ii) to determine whether infection control measures could potentially reduce the ESBL prevalence in Enterobacter species by assessing the association between prevalence and clonal relatedness of ESBL-positive isolates per hospital.For this study, all Enterobacter blood culture isolates (one per patient) obtained in 2006 and 2007 from 12 Dutch laboratories were included. Identification was performed by the participating laboratories using either Vitek 2 (bioMérieux, France) or Phoenix (Becton Dickinson). A total of 271 blood culture isolates were included; 227 (84%) were Enterobacter cloacae and 44 (16%) were Enterobacter aerogenes. Based on detection of TEM, SHV, and CTX-M ESBL genes by sequencing, 37 (14%) were ESBL producers (36 E. cloacae isolates and 1 E. aerogenes isolate). The most prevalent ESBL genes were CTX-M-9 (54%) and SHV-12 (38%), in accordance with results from previous studies (1, 7, 9, 13). CTX-M-15, CTX-M-3, and CTX-M-39 were each detected once.To develop a phenotypic ESBL detection strategy, the Vitek 2 advanced expert system (AES) (version 5.01; AST N...

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