 An alternative theory suggests that the symptoms of the disease may be caused by toxic effects of the hydrogen sulfide on the cells lining the intestine.Infection by mycobacterium avium, subspecies paratuberculosis, has been proposed as the ultimate cause of both ulcerative colitis and Crohn's disease.An increased amount of colonic sulfate-reducing bacteria has been observed in some people with ulcerative colitis, resulting in higher concentrations of the toxic gas hydrogen sulfide.