The relationship between human diseases caused by infection with Shiga toxin (Stx)-producing Escherichia coli (STEC) O157 strains and O157 strains isolated from cattle was investigated in an area where stockbreeding is prolific. For this purpose, the stx genotypes, the molecular epidemiological characteristics of 268 STEC O157 strains including 211 human-origin strains and 57 cattle-origin strains, and clinical manifestations of 210 STEC-infected people were analyzed. Of 211 human-origin strains, 92 strains (44%) were of the stx1/stx2 genotype, and 74 strains (35%) were of the stx2c genotype. Most of the people infected with stx2c genotype strains presented no symptoms or mild symptoms such as slight diarrhea, except for 3 patients with bloody diarrhea. Of the 57 cattle-origin strains, 27 strains (47%) were of the stx2c genotype and 17 strains (30%) were of the stx1/stx2 genotype. Pulsed-field gel electrophoresis (PFGE) and insertion sequence (IS) analysis demonstrated that 11 isolates (41%) of the 27 cattle isolates of the stx2c genotype had high homology (>95% identity) with human isolates. These results suggest that some genetic patterns of the stx2c genotype strains might be preserved in cattle or their surrounding environment for several years, and during these periods, they might have opportunities to infect people through various routes. Because of the mild virulence of the stx2c genotype strains, they seemed to be transmitted asymptomatically from cattle to humans and then spread from person to person. It may be a public health concern. Further, they occasionally cause severe symptoms in humans; therefore, caution is warranted for infections by stx2c genotype O157 strains, in addition to stx2-possessing genotype O157 strains.

SUMMARY:To determine the expression level of Shiga toxin (Stx) 2-related toxins (Stx2 and Stx2c) produced by each of 33 Stx-producing Escherichia coli (STEC) O157 strains, stx2 and stx2c mRNAs (stx2-related mRNA) were measured using real-time PCR with primers that recognize sequences common to stx2 and stx2c. The amount of Stx2 and Stx2c protein was measured using a reversed passive latex agglutination (RPLA) kit. Expression of stx2-related mRNA was significantly higher in STEC O157 strains carrying the stx2 gene (i.e., stx2, stx1/stx2, or stx2/stx2c) than in most strains that carried the stx2c gene but not the stx2 gene (i.e., stx2c or stx1/stx2c). RPLA might not measure the precise amount of each toxin variant; nevertheless, stx2-inclusive strains had 40-fold higher mean toxin titers than did strains that carried the stx2c gene but not the stx2 gene, with the exception of 1 stx2c strain. Interestingly, 1 stx2c strain that was isolated from a patient with severe hemorrhagic diarrhea had the highest stx2-related mRNA expression and the highest toxin titer of all 33 STEC O157 strains. Taken together, these findings indicated that measurement of stx2-related mRNA expression could reflect differences in production levels of toxins among STEC strains.Shiga toxin (Stx)-producing Escherichia coli (STEC) are emerging pathogens of significant clinical and public health concern that can cause symptoms ranging from mild to severe, including diarrhea, hemorrhagic colitis, and hemolytic-uremic syndrome, in humans (1). A STEC strain can produce any one of several Stx proteins or some combination of these. These proteins include Stx1, Stx1c, Stx1d, Stx2, Stx2c, Stx2d, Stx2e, Stx2f, and Stx2g, and these toxins are encoded by the stx1, stx1c, stx1d, stx2, stx2c, stx2d, stx2e, stx2f, and stx2g genes, respectively. In Japan, O157 is the dominant serogroup among STEC of human origin, and most O157 isolates from humans produce Stx1, Stx2, Stx2c, or some combination of these 3 Stx variants (2-5). In our previous study, we demonstrated that each of 268 STEC O157 strains of human (211 strains) or cattle (57 strains) origin had 1 of the following 6 genotypes: stx1 alone (hereafter, stx1), stx2 alone (hereafter, stx2), stx2c alone (hereafter, stx2c), both stx1 and stx2 (hereafter, stx1/stx2), both stx1 and stx2c (hereafter, stx1/stx2c), or both stx2 and stx2c (hereafter, stx2/stx2c) (3). Of these 6 genotypes, only 5 (stx1 excluded) were detected among human isolates, and 5 genotypes (stx2/stx2c excluded) were detected among cattle isolates.Siegler et al. and Boerlin et al. showed that enteric infection by Stx2-producing E. coli was more likely to cause severe human disease than infection by organisms that produce only Stx1 (6,7). Furthermore, most STEC O157 strains carrying the stx2 gene (i.e., stx1/stx2, stx2, and stx2/stx2c) are more virulent than strains that do not carry the stx2 gene (i.e., stx1, stx1/stx2c, and stx2c) (2-5); therefore, it is reasonable to speculate that the severity of STEC O157-related diseases in humans may be a...