VIRULENCE PROPERTIES OF SHIGA TOXIN-PRODUCING ESCHERICHIA-COLI (STEC)STRAINS OF SEROGROUP O118, A MAJOR GROUP OF STEC PATHOGENS IN CALVES

Shiga toxin-producing Escherichia coil (STEC) strains of serogroup O11 8 are the most prevalent group among STEC strains in diarrheic calves in Germany (L. H. Wieler, Ph.D. thesis, University of Giessen, 1997). To define their virulence properties, 42 O118 (O118:H16 [II = 38] and O118:H- [n = 4]) strains were characterized. The strains displayed thr ee different Stx combinations (Stx1 [36 of 42], Stx1 and Stx2 [2 of 42 ], and Stx2 [4 of 42]). A total of 41 strains (97.6%) harbored a large virulence-associated plasmid containing hly(EHEC) (hly from enterohem orrhagic E. coil). The strains' adhesive properties varied in relation to the eukaryotic cells tested. Only 28 of 42 strains (66.7%) showed localized adhesion (LA) in the human HEp-2 cell line. In contrast, in bovine fetal calf lung (FCL) cells, the number of LA-positive strains was much higher (37 of 42 [88.1%]). The locus of enterocyte effacement (LEE) was detected in 41 strains (97.6%). However, not all LEE-positi ve strains reacted positively in the fluorescence actin-staining (FAS) test, which indicated the attaching and effacing (AE) lesion. In HEp- 2 cells, only 22 strains (52.4%) were FAS positive, while iu FCL cells , the number of FAS-positive strains was significantly higher (38 of 4 2 [90.5%; P < 0.001]). In conclusion, the vast majority of the O118 ST EC strains from calves (41 of 42 [97.6%]) have a high virulence potent ial (stx, hly(EHEC), and LEE). This virulence potential and the high p revalence of STEC O118 strains in calves suggest that these strains co uld be a major health threat for humans in the future. In addition, th e poor association between results of the geno- and phenotypical tests to screen for the AE ability of STEC strains calls the diagnostic val ue of the FAS test into question.