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Extended-spectrum beta-lactamase (ESBL)-producing Escherichia coli strains are emerging pathogens. Molecular typing of ESBL-producing E. coli is useful for surveillance purposes, to monitor outbreaks and track nosocomial spread. Although pulsed-field gel electrophoresis (PFGE) is the current "gold standard" for bacterial molecular typing, multilocus sequence typing (MLST) may offer advantages. Forty ESBL-producing E. coli isolates were selected at random from a cohort of intensive care unit patients who had active surveillance perirectal cultures done. PFGE identified 19 unique PFGE types (PT) among the 40 isolates; MLST identified 22 unique sequence types. MLST had greater discriminatory ability than PFGE for ESBL-producing E. coli. Simpson's indices of diversity for PFGE and MLST were 0.895 and 0.956, respectively. There were five clonal complexes (CCs) (isolates with differences of no more than two loci) that each contained multiple PT, but each PT was found in only one CC, indicating genetic consistency within a CC. MLST has clear utility in studies of ESBL-producing E. coli, based on a greater discriminatory ability and reproducibility than PFGE and the ability to a priori define genetically related bacterial strains.Pulsed-field gel electrophoresis (PFGE) is the most widely used tool for molecular typing of bacterial strains (1), but new DNA fingerprinting techniques, including multilocus sequence typing (MLST), are emerging as alternatives, particularly when information regarding evolutionary history is needed (30). MLST is a relatively new technique where multiple genes (loci) are sequenced to measure genetic relatedness and analyze sequence variation between alleles from many strains (20). MLST has already been used to characterize a number of pathogenic bacteria, including Neisseria meningitidis, Enterococcus spp., Staphylococcus aureus, and Campylobacter spp. (3,4,7,21). In studies comparing MLST to PFGE among Salmonella spp., Vibrio cholerae, Enterococcus faecalis, and S. aureus isolates, MLST has been found to have similar or greater discriminatory ability than PFGE (17,18,21,27). However, these results may not be generalizable to all species, since a study comparing MLST to PFGE in Escherichia coli O157:H7 found that PFGE had a greater discriminatory ability than MLST did (26).MLST has not yet been used to characterize extended-spectrum beta-lactamase (ESBL)-producing E. coli isolates. ESBL production, which confers resistance to all penicillins and cephalosporins, is found in E. coli and other members of the family Enterobacteriaceae. In the United States, the prevalence of ESBL carriage ranges between 0 and 25%, with a national average of approximately 6%, and these numbers are increasing (2, 24). ESBL-producing E. coli bacteria cause nosocomial infections, including bloodstream infections, urinary tract infections, and pneumonias, and are associated with increased patient morbidity and mortality (29). Molecular typing of ESBL-producing E. coli is useful as a framework for hospital epidemiologist...

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