Virulence factors
Among the 56 STEC isolates, 31 (55.4%) were positive for stx 1 only, 3 (5.4%) for stx 2 only, and 22 (39.3%) forstx 1+stx 2. Stx 1 was subtyped into stx 1a (35/53=66%) and stx 1c (18/53=34%), and stx 2 intostx 2a (14/25=56.0%), stx 2b (5/25=20.0%), andstx 2c (6/25=24.0%). These stx genes gave rise to 8 subtype combinations (stx 1a, stx 1c, stx 2a,stx 2b, stx 2c, stx 1a+stx 2a,stx 1a+stx 2c, and stx 1c+stx 2b), withstx 1a being the most prevalent (17/56=30.4%). The prevalence of other subtype combinations was 25.0% (14/56, stx 1c), 23.2% (13/56, stx 1a+stx 2a), 8.9% (5/56,stx 1a+stx 2c), and 7.1% (4/56, stx 1c+stx 2b). Two subtypes, stx 1c and stx 2b, are exclusively associated with sheep isolates, whereas stx 2c was exclusively related to cattle isolates. Cattle isolates are more likely to have thestx 1+stx 2 combination (17/27= 63.0%) than the sheep isolates (5/29= 17.2%). Stx 1-only genotype is more prevalent in sheep isolates (23/29=79.3%) than in cattle isolates (8/27=29.6%) (Fig. 1).
None of the isolates was positive for eae as well as adhesinstoxB and efa1 (data not shown). Iha , which encodes an adherence-conferring protein similar to irgA of Vibrio cholerae , is present in a majority of strains (32/56=57.1%).Saa , which encodes an autoagglutinating adhesin that is usually identified in LEE-negative strains, is present in 11 cattle strains (11/27=40.7%), but not in the sheep strains. In contrast, ireAis only encoded by sheep isolates (16/29=55.2%), but not cattle isolates. Nineteen strains carried subA , and sheep strains apparently had a higher percentage (16/29=55.2%) compared to cattle strains (3/27=11.1%). Hemolysin-encoding gene ehxA is present in 26 strains (46.4%), which is concordant to the findings of previous studies that show that ehxA is present in about half of non-O157 STEC strains (Hussein & Bollinger, 2005). Only cattle isolates possess the espP gene (7/27=25.9%).Irp2 and fyuA carried by high pathogenicity island (HPI) were found in 13 isolates (23.2%). Eight strains (14.3%) harboredkatP (Fig. 1).