Suppression of Potential Pathogens by a Defined Colonic Microflora

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Abstract
Animals acquiring a microflora for the first time do so through the gradual process of ecologic succession. A defined microflora was derived by experimentally simulating this process in gnotobiotic mice. Diverse bacterial species were obtained from ex-germfree mice acquiring a microflora from a conventional mouse. All isolates were characterised to the genus level and polyacrylamide gel electrophoresis (PAGE) was performed on soluble proteins to differentiate isolates at the species or subspecies level. Selected isolates were fully identified. Only 21 per cent of taxa had been previously described. Organisms isolated early in succession tended to ferment more diverse carbohydrates (mean ± SD = 10·2 ± 8·6 carbohydrates per isolate) than organisms found in climax stage mouse flora (5·7 ± 7·0). Bacterial species which predominated during ecologic succession, but which later comprised only a small part of the microflora, were important in simulating certain functions of the entire flora. The defined flora suppressed Escherichia coli to 106·6 colony forming units (CFU) per caecum, a degree of suppression similar to that caused by the entire caecal flora. Pseudomonas aeruginosa was suppressed less by the defined flora (104·4) than by the entire caecal flora (102·0). While Clostridium difficile was suppressed much less by the defined flora (106·6) than by the entire flora (undetectable), animals colonised with the defined flora were protected from colitis when challenged with a pathogenic strain of C. difficile.