Methods and compositions for the identification of functional C. difficile toxin, as among other things identifying individuals infected with toxigenic C. difficile and therefore in need of therapy. In specific embodiments, the methods and compositions provide colorimetric assays for cleavage activity of C. difficile toxin.

Described are an oligo-peptide sequence of any of RDHLVGRWIR (E1), IRRFKSSKFGCR (E2), RHLEIIYAINQR (E3), LIIKLVT (E4), VVGDRLKVIF (E5), any combination of E1 to E3; or combination of E4 and E5. Further described is a detecting agent for specific detecting glycogen phosphorylase iso-enzyme BB (GPBB), wherein the detecting agent is characterized by specific recognizing and binding to (1) an epitope of the GPBB comprising an oligo-peptide sequence of any of RDHLVGRWIR (E1), IRRFKSSKFGCR (E2), RHLEIIYAINQR (E3) or any combination of E1 to E3; or (2) an epitope of GPBB comprising an oligo-peptide sequence of LIIKLVT (E4), and/or VVGDRLKVIF (E5), or combination of E4 and E5.