Data suggest that altered gut microbial populations may be partially responsible for the development of HAEC in susceptible HD patients. Most studies have focused on Clostridium difficile, because two groups reported increased frequency of C. diff. toxin positive stools in HD patients with HAEC compared with those without HAEC .[10, 11] Other groups subsequently reported very low frequencies of C. diff. toxin positivity in their patients with HAEC, thereby calling into question the role C. diff. plays in the pathogenesis of HAEC.[12] Other investigators have found E. coli, C. diff. and Cryptosporidium adherent to enterocytes on histological examinations of colon biopsies of patients with HAEC.[5] These findings indicate a breech in the protective mucus gel layer covering the luminal surface of the colon, leading to invasion of the epithelial barrier. Taken together, these studies suggest that the intestine-microbial interaction may be important in the pathogenesis of HAEC.