Differential expression of stx2 variants in Shiga toxin-producing Escherichia coli belonging to

Only a subset of Shiga toxin (Stx)-producing Escherichia coli (STEC) are human pathogens, but the characteristics that account for differences in pathogenicity are not well understood. In this study, we investigated the distribution of the stx variants coding for Stx2 and its variants in highly virulent STEC of seropathotype A and low-pathogenic STEC of seropathotype C. We analysed and compared transcription of the corresponding genes, production of Shiga toxins, and stx-phage release in basal as well as in induced conditions. We found that the stx2 variant was mainly associated with strains of seropathotype A, whereas most of the strains of seropathotype C possessed the stx2-vhb variant, which was frequently associated with stx2, stx2-vha or stx2c. Levels of stx2 and stx2-related mRNA were higher in strains belonging to seropathotype A and in those strains of seropathotype C that express the stx2 variant than in the remaining strains of seropathotype C. The stx2-vhb genes were the least expressed, in basal as well as in induced conditions, and in many cases did not seem to be carried by an inducible prophage. A clear correlation was observed between stx mRNA levels and stx-phage DNA in the culture supernatants, suggesting that most stx2-related genes are expressed only when they are carried by a phage. In conclusion, some relationship between stx2-related gene expression in vitro and the seropathotype of the STEC strains was observed. A higher expression of the stx2 gene and a higher release of its product, in basal as well as in induced conditions, was observed in pathogenic strains of seropathotype A. A subset of strains of seropathotype C shows the same characteristics and could be a high risk to human health