PRB2 and clostridium difficile infection: So far, many surface components have been evaluated, which include surface proteins and colonization factors, such as SlpA, Cwp84, Cwp66, FliC, FliD, CdeC, CdeM, cell wall polysaccharides (PS-I, PS-II, PS-III), lipoprotein CD0873, chaperon protein DnaK, heat shock protein GroEL, BclA3, etc., all of which can induce protective immune responses against CDI in animals to different extents; however, none of them have been evaluated in clinical trials [37,50].