Structural and functional characterization of Stx2k, a new subtype of Shiga toxin 2

Authors: Hughes, Anna; Zhang, Yuzhu; BAI, XIANGNING - Chinese Center For Disease Control; XIONG, YANWEN - Chinese Center For Disease Control; WANG, YAN - Chinese Center For Disease Control; YANG, XI - Chinese Center For Disease Control; XU, QINGPING - Chinese Center For Disease Control; He, Xiaohua

Submitted to: Microorganisms
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/14/2019
Publication Date: 12/18/2019
Citation: Hughes, A.C., Zhang, Y., Bai, X., Xiong, Y., Wang, Y., Yang, X., Xu, Q., He, X. 2019. Structural and functional characterization of Stx2k, a new subtype of Shiga toxin 2. Microorganisms. 8(1):4. https://doi.org/10.3390/microorganisms8010004.
DOI: https://doi.org/10.3390/microorganisms8010004

Interpretive Summary: Infection with Shiga toxin (Stx)-producing Escherichia coli (STEC) can cause severe human illnesses including mild diarrhea to the potential life threatening hemolytic uremic syndrome. While Stx produced by STEC strains are most closely associated with these diseases. Recently, we identified a new Stx2k, which was detected poorly by existing test kits and no structural and biological information is available. In this study, we determined the crystal structure of the Stx2k, developed a novel antibody and immunoassay for the toxin and investigated the cytotoxicity, thermal stability, and acid tolerance of the Stx2k. The information and tools are valuable for diagnosis of infections caused by emerging STEC strains that produce less common Stxs, therefore, reducing product recalls and disease mistreatment.

Technical Abstract: Shiga toxin (Stx) is the major virulence factor of Shiga toxin-producing Escherichia coli (STEC). Stx are diverse, immunologically variable, and rapidly evolving and as such new subtypes continue to emerge, challenging the efficacy of existing disease management and surveillance strategies. A new subtype, Stx2k, was recently identified, which was detected poorly by commercial immunoassays. To better characterize it, the structure of the Stx2k at 2.29 Å resolution was determined and reveals that the active site is similar to that of Stx2a. A novel polyclonal antibody and immunoassay were developed with a limit of detection of 35 pg/mL for Stx2k, a 100-fold increase in sensitivity compared to assays using extant Stx2a antibodies. Stx2k purified from a clinical STEC strain was evaluated with Vero cell assays. Stx2k had a CD50 100 pg/well, a 20-fold less toxic than Stx2a. It was like Stx2a in receptor-binding preference for Gb3, thermostability and acid tolerance. Stx2k was completely abrogated when it was heated to 95C, but it retained 40% of cytotoxicity at 74C. No detrimental effect was observed on Stx2k stability and cytotoxicity at pH 3 and above. Although Stx2k does not appear to be as potent as Stx2a, the wide distribution and extraordinary plasticity of the Stx2k-producing strains could provide opportunities for Stx2k-converting phages to infect a broader range of bacteria, resulting in emergence of new pathogens with high virulence. This study provides useful information and tools for early detection and control of Stx2k-producing E. coli, and therefore, reducing public risk of STEC infections.