Switching Shiga Toxin (Stx) Type from Stx2d to Stx2a but Not Stx2c Alters Virulence of Stx-Producing Escherichia coli (STEC) Strain B2F1 in Streptomycin (Str)-Treated Mice.

Beth A Mcnichol,Lan N Preston,Angela R Melton-Celsa,Rebecca A Bova,Kieron Torres

doi:10.3390/toxins13010064

Abstract

Shiga toxin (Stx)-producing Escherichia coli (STEC) strain B2F1 produces Stx type 2d, a toxin that becomes more toxic towards Vero cells in the presence of intestinal mucus. STEC that make Stx2d are more pathogenic to streptomycin (Str)-treated mice than most STEC that produce Stx2a or Stx2c. However, purified Stx2d is only 2- or 7-fold more toxic by the intraperitoneal route than Stx2a or Stx2c, respectively. We hypothesized, therefore, that the toxicity differences among Stx2a, Stx2c, and Stx2d occur at the level of delivery from the intestine. To evaluate that hypothesis, we altered the toxin type produced by stx2d+ mouse virulent O91:H21 clinical isolate B2F1 to Stx2a or Stx2c. Because B2F1 encodes two copies of stx2d, we did these studies in a derivative of B2F1 in which stx2d1 was deleted. Although the strains were equivalently virulent to the Str-treated mice at the 1010 dose, the B2F1 strain that produced Stx2a was attenuated relative to the ones that produced Stx2d or Stx2c when administered at 103 CFU/mouse. We next compared the oral toxicities of purified Stx2a, Stx2c, and Stx2d. We found that purified Stx2d is more toxic than Stx2a or Stx2c upon oral administration at 4 µg/mouse. Taken together, these studies suggest that Stx2 toxins are most potent when delivered directly from the bacterium. Furthermore, because Stx2d and Stx2c have the identical amino acid composition in the toxin B subunit, our results indicate that the virulence difference between Stx2a and Stx2d and Stx2c resides in the B or binding subunit of the toxins.

Highlights

Shiga toxin (Stx)-producing Escherichia coli (STEC) are foodborne causes of bloody diarrhea, and infection may lead to a serious sequela, hemolytic uremic syndrome (HUS), which is characterized by hemolytic anemia, thrombocytopenia, and renal failure
In this study we switched the toxin type produced by STEC strain B2F1 from Stx2d to Stx2a or Stx2c to determine if Stx2d was the sole determinant of the high pathogenicity of B2F1 in Str-treated mice, and to evaluate if these toxins were of similar potency as delivered by an STEC from the intestine
Our study suggests that Stx2d and Stx2c are equivalently toxic when produced from

Summary

Introduction

Shiga toxin (Stx)-producing Escherichia coli (STEC) are foodborne causes of bloody diarrhea, and infection may lead to a serious sequela, hemolytic uremic syndrome (HUS), which is characterized by hemolytic anemia, thrombocytopenia, and renal failure. STEC that make the activatable Stx2d are more pathogenic to streptomycin (Str)-treated mice than most STEC that produce the non-activatable Stx2a or Stx2c [5,6]. Purified Stx2d is only 2- or 7-fold more toxic to mice by the intraperitoneal (ip) route than Stx2a or Stx2c, respectively [6,7]. That the toxicity differences among Stx2a, Stx2c, and Stx2d occur at the level of delivery from the intestine. In this study we switched the toxin type produced by STEC strain B2F1 from Stx2d to Stx2a or Stx2c to determine if Stx2d was the sole determinant of the high pathogenicity of B2F1 in Str-treated mice, and to evaluate if these toxins were of similar potency as delivered by an STEC from the intestine

Methods

Results

Conclusion