The physiological importance of the colonic microflora

The present concept of the intestinal microflora has been reviewed with stressing the fact that it represents an entity with a weight comparable to one of the larger organs of the body. It is composed of about 500 different strains, many of which as yet have not been isolated because 95% of them are anaerobic and fastidious in their growth requirements. The intimate relationship of the flora to the surface epithelium of the intestines and the Lieberkühn's crypts are brought forward. The production of germfree animals is reviewed as an accounting of the most obvious differences between the germfree animal and its conventional counterparts, as far as gastroenterology is concerned. This includes the protective function of the normal intestinal flora, the caecum enlargement in germfree rodents, the influence of the flora on the metabolism of bilirubin, intestinal mucin, and pancreatic enzymes. Differences found in germfree animals due to the absence of the microbial flora related to the metabolism of fatty acids, bile acids, cholesterol and steroid hormones are also reviewed. The germfree animal has even proven to be an important research tool in such fields as carcinogenesis and studies of Crohn's disease and ulcerative colitis. There is need of a wider use of the germfree animal as a baseline in studies on the interference of the intestinal microbial flora with factors and conditions of great physiological and clinical importance.