Characterization of cefotaxime-resistant Escherichia coli isolates from a nosocomial outbreak at three geriatric hospitals.

Over a 22-month period, there was an unusual upsurge in the incidence of cefotaxime-resistant Escherichia coli among hospitalized patients in three geriatric hospitals in the same district. Sixteen highly cefotaxime-resistant strains were obtained from clinical specimens during the period January 1996 through October 1997. All strains were characterized by antibiotic resistance pattern analysis, the detection of the TEM- and Toho-type beta-lactamase or CTX-M-type beta-lactamase gene by polymerase chain reaction (PCR), plasmid profiling, Southern hybridization analysis, and pulsed-field gel electrophoresis (PFGE). Antibiotic resistance analysis showed that all strains were highly resistant to ampicillin, piperacillin, carbenicillin, cephaloridine, and cefotaxime; intermediately resistant to cefoxitin; moderately susceptible to moxalactam and ceftazidime; and susceptible to imipenem. Detailed analysis of beta-lactamase content revealed that all cefotaxime-resistant strains harbored a plasmid that mediated an extended-spectrum beta-lactamase of the Toho-type or CTX-M-type by PCR and Southern hybridization analysis. PCR detection showed that all the E. colistrains, except for strains TUM1023, TUM1101, TUM1227, and TUM1229, also possessed bla(TEM) genes. Furthermore, Southern hybridization analysis showed that all strains, except for TUM1102, gave a similar signal with the Toho probe. The PFGE profiles of the E. colistrains obtained with XbaI showed four patterns that correlated well with the plasmid profiles. The Dice value of 15 strains, including Toho-2 producer (TUM1083), for their PFGE patterns indicated a similarity of 80% or more. Our results suggest that 15 of the 17 Toho type beta-lactamase-producing E. coli strains (including strain TUM1083) studied belong to a single epidemic strain, while the other two strains are different from them, and the Toho-type or CTX-M-type beta-lactamase encoding gene may be acquired by plasmid conjugation or a mobile element.