Assays for Measuring C. difficile Toxin Activity and Inhibition in Mammalian Cells

Abstract

Abstract Clostridium difficile infections (CDIs) are the leading cause of hospital-acquired infectious diarrhea. The symptoms of CDI are caused by two exotoxins, TcdA and TcdB, which are structurally and functionally highly homologous. Both toxins bind to specific receptors on mammalian cells, are internalized through endocytosis, translocate to the cytoplasm, and inactivate Rho-type GTPases via covalent glucosylation. This leads to downstream events that include morphological changes and disruption of epithelial tight junctions, release of pro-inflammatory mediators, and cell death. Assays used to assess the effects of toxins on cells have historically relied on evaluation of cell rounding or quantitation of ATP levels to estimate cell death—assays which can be qualitative and variable. In this chapter, several assays are described that robustly and quantitatively measure early and late toxin-dependent events in cells, including (i) toxin binding, (ii) Rac1 glucosylation, (iii) changes in cellular morphology (measured as dynamic mass redistribution), (iv) loss of epithelial integrity (measured as transepithelial electrical resistance), and (v) cell death (measured as total cellular protein using a colorimetric assay). The assays were validated using the highly specific monoclonal antitoxin antibodies, actoxumab and bezlotoxumab, which neutralize TcdA and TcdB, respectively.