Characterization of Shiga toxin-producing Escherichia coli strains isolated from swine feces.

The virulence gene and antibiotic resistance profiles of Shiga toxin-producing Escherichia coli (STEC) strains belonging to 58 different O:H serotypes (219 strains) isolated from swine feces were determined. Of the 219 isolates, 29 (13%) carried the stx(1) gene, 14 (6%) stx(2), 176 (80%) stx(2e), 46 (21%) estIa, 14 (6.4%) estIb, 10 (4.6%) fedA, 94 (42.9%) astA, 25 (11.4%) hly(933), and one (0.46%) cdt-III. None of the strains possessed the elt, bfp, faeG, fanA, fasA, fimF(41a), cnf-1, cnf-2, eae, cdt-I, or cdt-IV genes. The strains were also tested for antibiotic susceptibility using 16 antibiotics. The STEC isolates displayed resistance most often to tetracycline (95.4%), sulfamethoxazole (53.4%), kanamycin (38.4%), streptomycin (34.7%), and chloramphenicol (22.4%). An E. coli serotype O20:H42 strain, which was positive for stx(2e) and astA, was resistant to all of the antibiotics tested except for amikacin. In addition, 52 of the swine isolates, representing 16 serogroups and 30 different serotypes, were examined for their ability to withstand acid challenge by three types of acid resistance (AR) pathways, AR1 (rpoS dependent), AR2 (glutamate dependent), and AR3 (arginine dependent). None of the strains was defective in the AR1 resistance pathway, while one strain was defective in the AR2 pathway under aerobic growth conditions but fully functional under anaerobic growth conditions. Among the three AR pathways, the AR3 pathway offered the least protection, and 8 out of 52 strains were defective in this pathway. The strain that was defective in AR2 was fully functional in the AR3 pathway. Since AR plays a vital role in the survival and virulence of these strains, differences among the isolates to induce AR pathways may play a significant role in determining their infective dose. This study demonstrates that swine STEC comprise a heterogeneous group of organisms, and the possession of different combinations of E. coli virulence genes indicate that some swine STEC can potentially cause human illness.