Visualizing the Effects of Clostridium difficile Toxins A and B on Mammalian Epithelial Cells

Clostridium difficile is a spore-forming, obligate anaerobe, Gram-positive bacterium that causes Clostridium difficile infection (CDI), responsible for ~30,000 deaths annually. C. difficile secretes Toxin A and Toxin B, closely related proteins which target host small GTPases and have been associated with severe symptoms of CDI. We studied the effects of these toxins on epithelial cell junctions and actin cytoskeletons via live-cell fluorescent microscopy. Epithelial cells form a protective barrier by generating tight, strong junctions on all sides. The actin cytoskeleton allows cells to bind to the substrate and to interact with their neighboring cells. Using real-time, high-resolution fluorescent microscopy we show how C. difficile affects host cell junctions and cytoskeleton. Additionally, we tested the effects of purified toxins at various concentrations on these fluorescently labeled cells to clarify the role of each toxin separately during CDI. Toxins A and B appear to work on different timescales and have different effects on epithelial cell physiology. Cells introduced to Toxin A retracted significantly yet maintained cell junctions relatively well. Cells treated with Toxin B rounded more gradually. Treatment with Toxin A+B caused dramatic rounding and degradation of cells junctions compared to healthy controls. In addition, the tight junctions showed distress slightly sooner than the actin cytoskeleton. Toxin concentration determined how quickly the epithelial cells showed damage, but all concentrations tested were eventually fatal, illustrating how potent these toxins are against mammalian cells. These findings improve our mechanistic understanding of infection bringing us one step closer to effective treatment