To the Editor: Since the early 1980s, extended-spectrum β-lactamases (ESBLs) have been the largest source of resistance to broad-spectrum oxyimino-cephalosporins among Enterobacteriaceae (1). Molecular analysis techniques suggest that many ESBLs are derived from mutations in TEM-1, TEM-2, and SHV-1 β-lactamases and that these ESBLs can hydrolyze the extended-spectrum cephalosporins (particularly ceftazidime) and aztreonam (1). Members of a new group of ESBLs have been recently identified (1). Among them, CTX-M-type ESBLs are rapidly expanding and are derived from chromosomal class A β-lactamases of Kluyvera spp. (1, 2) . The CTX-M enzymes are not related to TEM or SHV enzymes, as they share only 40% identity with these ESBLs (2) .These ESBLS are usually characterized by a higher level of resistance to cefotaxime than ceftazidime, except for CTX-M-19 (2) . Most organisms that harbor ESBLs are also resistant to other classes of antimicrobial drugs, such as aminoglycosides, fluoroquinolones, chloramphenicol, and tetracyclines (1, 2) .