Phylogenetic analysis and Shiga toxin production profiling of Shiga toxin-producing/enterohemorrhagic Escherichia coli clinical isolates

Shiga toxin-producing Escherichia coli (STEC) can cause severe illnesses in humans such as hemorrhagic colitis and hemolytic-uremic syndrome. In this study, we carried out genotypic analysis of the Shiga toxin (stx) gene in 120 clinical isolates of STEC and enterohemorrhagic E. coli (EHEC) from patients in a southern district of Japan. We identified 88 stx1+ and 103 stx2+ strains. We further identified 12 stx1+ and stx2+ isolates expressing little or no Shiga toxin 1 (Stx1) and/or 2 (Stx2) by reversed passive latex agglutination (RPLA) and Vero cell toxicity assays. Among them, 1 strain could not produce Stx1, 8 could not produce Stx2, and 3 strains could produce neither. Two of the latter three strains were of the non-O157 serotype. Most of the Stx RPLA-negative strains belonged to the stx1/stx2 subtype (11/12, [91.7%]). Our quantitative reverse transcription PCR analysis indicated that the stx genes were not effectively transcribed in the RPLA-negative strains. This is the first report of the isolation of stx-positive strains showing Stx-negative phenotype from stx1-bearing strains and non-O157 strains.