Genotype, serotype, and antibiotic resistance of sorbitol-negative Escherichia coli isolates from feedlot cattle.

Rectal fecal samples from 80 steers receiving Rumensin, Revalor-S, and Liquamycin alone or in combination for growth promotion and disease prevention were examined for the presence of non-O157:H7 Shiga toxin-producing Escherichia coli. All isolates were identified with the API 20E test, virulence genes were detected with a PCR assay, and antibiotic susceptibilities were determined with the Sensititre system. Of the 153 E. coli isolates recovered 126 (82.3%) were sorbitol negative. Isolates were classified into 14 biochemical E. coli groups; 51.6% were negative for arginine dihydrolase, ornithine decarboxylase, sorbitol, and saccharose reactions but positive for lysine decarboxylase, indole production, and rhamnose reactions. Twenty-one O:H serotypes were detected in the 153 E. coli isolates. The most frequent serotypes were O2:H42 (49.7% of isolates), O49:NM (13.7%), O?:H25 (9.2%), and O10:NM (7.2%). One isolate of E. coli O172:H25 and one of E. coli O157: H39 were found. The stx1 gene was found in the two E. coli O98:H25 isolates. The eaeA and e-hlyA genes were detected in 21, 14, and 10 isolates of serotypes O49:NM, O?:H25, and O10:NM, respectively, and in each isolate of serotype O156:H25 and O172:H25. Four E. coli O132:H18 isolates were multiresistant to ampicillin, chloramphenicol, kanamycin, streptomycin, and sulfisoxazole. Tetracycline resistance due to the tet(B) gene was observed in 74 of the 76 E. coli O2:H42 isolates. Except for one isolate, all tetracycline-resistant isolates were negative for the virulence genes eaeA and e-hlyA or stx1. Pulsed-field gel electrophoresis typing revealed that the tetracycline-resistant serotypes were genetically diverse. Our data illustrate that cattle are a potential source of some atypical antibiotic-resistant E. coli isolates that harbor virulence genes.