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.

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.

Bacillus anthracis protease InhA regulates BslA-mediated adhesion in human endothelial cells.

To achieve widespread dissemination in the host, Bacillus anthracis cells regulate their attachment to host endothelium during infection. Previous studies identified BslA (Bacillus anthracis S-layer Protein A), a virulence factor of B. anthracis, as necessary and sufficient for adhesion of vegetative cells to human endothelial cells. While some factors have been identified, bacteria-specific contributions to BslA mediated adhesion remain unclear. Using the attenuated vaccine Sterne 7702 strain of B. anthracis, we tested the hypothesis that InhA (immune inhibitor A), a B. anthracis protease, regulates BslA levels affecting the bacteria's ability to bind to endothelium. To test this, a combination of inhA mutant and complementation analysis in adhesion and invasion assays, Western blot and InhA inhibitor assays were employed. Results show InhA downregulates BslA activity reducing B. anthracis adhesion and invasion in human brain endothelial cells. BslA protein levels in ΔinhA bacteria were significantly higher than wild-type and complemented strains showing InhA levels and BslA expression are inversely related. BslA was sensitive to purified InhA degradation in a concentration- and time-dependent manner. Taken together these data support the role of InhA regulation of BslA-mediated vegetative cell adhesion and invasion.

Two domains of cytotoxic necrotizing factor type 1 bind the cellular receptor, laminin receptor precursor protein.

Cytotoxic necrotizing factor type 1 (CNF1) and CNF2 are highly homologous toxins that are produced by certain pathogenic strains of Escherichia coli. These 1,014-amino-acid toxins catalyze the deamidation of a specific glutamine residue in RhoA, Rac1, and Cdc42 and consist of a putative N-terminal binding domain, a transmembrane region, and a C-terminal catalytic domain. To define the regions of CNF1 that are responsible for binding of the toxin to its cellular receptor, the laminin receptor precursor protein (LRP), a series of CNF1 truncated toxins were characterized and assessed for toxin binding. In particular, three truncated toxins, DeltaN63, DeltaN545, and DeltaC469, retained conformational integrity and in vitro enzymatic activity and were immunologically reactive against a panel of anti-CNF1 monoclonal antibodies (MAbs). Based on a comparison of these truncated toxins with wild-type CNF1 and CNF2 in LRP and HEp-2 cell binding assays and in MAb and LRP competitive binding inhibition assays and based on the results of confocal microscopy, we concluded that CNF1 contains two major binding regions: one located within the N terminus, which contained amino acids 135 to 164, and one which resided in the C terminus and included amino acids 683 to 730. The data further indicate that CNF1 can bind to an additional receptor(s) on HEp-2 cells and that LRP can also serve as a cellular receptor for CNF2.

A single amino acid substitution in the enzymatic domain of cytotoxic necrotizing factor type 1 of Escherichia coli alters the tissue culture phenotype to that of the dermonecrotic toxin of Bordetella spp.

Cytotoxic necrotizing factor type 1 (CNF1) and dermonecrotic toxin (DNT) share homology within their catalytic domains and possess deamidase and transglutaminase activities. Although each toxin has a preferred enzymatic activity (i.e. deamidation for CNF1 and transglutamination for DNT) as well as target substrates, both modify a specific glutamine residue in RhoA, Rac1 and Cdc42, which renders these GTPases constitutively active. Here we show that despite their similar mechanisms of action CNF1 and DNT induced unique phenotypes on HEp-2 and Swiss 3T3 cells. CNF1 induced multinucleation of HEp-2 cells and was cytotoxic for Swiss 3T3 cells (with binucleation of the few surviving cells) while DNT showed no morphological effects on HEp-2 cells but did induce binucleation of Swiss 3T3 cells. To determine if the enzymatic domain of each toxin dictated the induced phenotype, we constructed enzymatically active chimeric toxins and mutant toxins that contained single amino acid substitutions within the catalytic site and tested these molecules in tissue culture and enzymatic assays. Moreover, both site-directed mutant toxins showed reduced time to maximum transglutamination of RhoA compared with the parent toxins. Nevertheless, the substitution of threonine for Lys(1310) in the DNT-based mutant, while affecting transglutamination efficiency of the toxin, did not abrogate that enzymatic activity.