Shiga toxin activatable by intestinal mucus in Escherichia coli isolated from humans: predictor for a severe clinical outcome

BACKGROUND: Some Escherichia coli produce Shiga toxin (Stx) in which cytotoxicity is increased (activated) by intestinal mucus and elastase (Stx2d(activatable)). These strains are highly virulent in mice, but their association with human disease is poorly understood. We investigated the prevalence of Stx2d(activatable) among Stx-producing E. coli (STEC) isolated from humans and the association between production of this Stx and the clinical outcome of infection.

METHODS: A total of 922 STEC isolates obtained from patients with hemolytic uremic syndrome or bloody or nonbloody diarrhea or from asymptomatic carriers were tested for the gene encoding Stx2d(activatable) by PCR and PstI restriction analysis. The toxin activatibility by human and mouse intestinal mucus and by an elastase was determined by quantifying the cytotoxicity using the Vero cell assay.

RESULTS: The stx(2d-activatable) gene was identified in 60 (6.5%) of 922 STEC strains; in 31 of these strains, it was the sole stx gene. Thirty of these 31 strains produced Stx2d(activatable). All of them lacked the intimin-encoding eae gene. Among eae-negative STEC, which typically cause mild diarrhea or asymptomatic infection, production of Stx2d(activatable) was significantly associated with the ability to cause severe disease, including bloody diarrhea (P<.001), and with systemic complications, such as hemolytic uremic syndrome (P<.001).

CONCLUSIONS: Production of Stx2d(activatable) by the infecting STEC may predict a severe clinical outcome of the infection, with progression to hemolytic uremic syndrome. A prompt and comprehensive subtyping of stx genes in STEC isolates is necessary to alert the treating physician that a patient is at risk of developing hemolytic uremic syndrome, even though the infecting STEC lacks eae.