Clostridium difficile (C. difficile) DS1813 spores were provided by Les Baillie (Cardiff University). Spores were grown on BHIS agar at 37 • C for 4 days. The cells were collected, left overnight to sporulate, and then purified using density gradient centrifugation in 50 % sucrose as described previously . Spores were then washed in deionised water and stored at 4 • C.