CSTB and clostridium difficile infection: To determine if the small competitive advantage displayed by the ΔbcpO strain was in fact due to CDI, we competed ΔbcpO bacteria with ΔbcpAIOB mutant bacteria constitutively expressing the cognate bcpI gene (E264ΔbcpAIOB::bcpIE264) or a heterologous bcpI gene (E264ΔbcpAIOB::bcpIK96243).