Abstract
Genotypes of Shiga toxin-producing Escherichia coli (STEC) O157 isolated from humans and cattle were analyzed by uni- and multivariable logistic regression, and population structure methods, to gain insight into transmission and the nature of human infection. Eleven genotyping assays, including PCR typing of five virulence factors (stx(1), stx(2), stx(2c), eae, and ehxA) and a lineage-specific polymorphism assay using six markers (LSPA6), were considered in the analyses. The prevalence of the stx(1), stx(2), and stx(2c) virulence factors was significantly different between human and cattle isolates. However, multivariable regression revealed that the presence of only the stx(2) gene was significantly associated with human isolates after controlling for confounding effects. LSPA6 typing demonstrated an apparent difference in the distribution of LSPA6 lineages between human and cattle isolates and a strong association between stx genotypes and LSPA6 genotypes. Population genetics tools identified three genetically distinct clusters of STEC O157. Each cluster was characterized by stx genotypes and LSPA6 genotypes. The human isolates typically comprised LSPA6 lineage I with stx(1) stx(2) strains and LSPA6 lineage I/II with stx(2c) or stx(2) stx(2c) strains [corrected]. In contrast, the cattle isolates comprised LSPA6 lineage II strains withstx(2c) or stx(1) stx(2c) strains [corrected] in addition to the clusters identified for the human isolates. Our analyses provide new evidence that the stx(2) gene is the most distinctive feature in human isolates compared to cattle isolates in Japan, and only a subset of the genetically diverse population isolated from cattle is involved in human illnesses. Our results may contribute to international comparisons and risk assessments of STEC O157.