Phylogenetic analysis of Shiga toxin 1 and Shiga toxin 2 genes associated with disease outbreaks

Phylogenetic analysis was performed using Stx1 and Stx2 amino acid and nucleotide sequences from 41 strains of Escherichia coli, along with known stx sequences available from GenBank. The analysis confirmed the Stx1 and Stx2 divergence, and showed that there is generally more sequence variation among stx2 genes than stx1. The phylograms showed generally flat topologies among our strains' stx1 and stx2 genes. In the stx2 gene, 39.5% of the amino acid sites display very low nonsynonymous to synonymous substitution ratios.The stx1 and stx2 genes used in this phylogenetic study show sequence conservation with no significant divergence with respect to place or time. These data could indicate that Shiga toxins are experiencing purifying selection.Shiga toxin was discovered in Shigella dysenteriae serotype 1 by Kiyoshi Shiga in 1898 [1]. The cytotoxic effects of Escherichia coli-produced Shiga toxin on Vero cells were first described 29 years ago [2]. A few years later, the toxin was closely associated with hemorrhagic colitis, hemolytic uremic syndrome (HUS) and other severe disease conditions [3,4]. Shiga toxin producing E. coli have been implicated in food borne, waterborne and airborne outbreaks in studies all over the world [5-7]. Much of the focus of identification and characterization of Shiga toxin has been on E. coli O157:H7 strains, even though many cases of Shiga toxin associated disease were caused by other serotypes of E. coli [8]. The toxin has also been observed in other bacterial genera, including Citrobacter, Enterobacter and Acinetobacter [9-11].Shiga toxin 1 (Stx1) and Shiga toxin 2 (Stx2) are encoded on a lambdoid bacteriophage. Stx1 is genetically and immunologically distinct from Stx2, showing 55–60% genetic and amino acid identity [12]. Stx1 is very similar to the Shiga toxin (Stx) found in Shigella dysenteriae type 1 [13]. Several variants of Stx1 (Stx1c and Stx1d) and Stx2 (Stx2c, Stx2d, Stx2e, Stx2f, and Sxt2g) have been described [14-17]. Both St