A previous study had established that anaerobic continuous-flow (CF) cultures of conventional mouse cecal flora were able to maintain the in vivo ecological balance among the indigenous bacterial species tested. This paper describes experiments designed to determine the mechanisms which control the population sizes of these species in such CF cultures. One strain each of Escherichia coli, Fusobacterium sp., and Eubacterium sp. were studied. Growth of these strains in filtrates of CF cultures was considerably more rapid than in the CF cultures themselves, indicating that the inhibitory activity had been lost in the process of filtration. Growth rates to match those in CF cultures could be obtained, however, by restoring the original levels of H2S in the culture filtrates. The inhibitory effect of H2S in filtrates and in dialysates of CF cultures could be abolished by adding glucose or pyruvate, but not formate or lactate. The fatty acids present in CF cultures matched those in the cecum of conventional mice in both quality and concentration. These acids could not account for the slow rates of growth of the tested strains in CF cultures, but they did cause a marked increase in the initial lag phase of E. coli growth. The results obtained are compatible with the hypothesis that the populations of most indigenous intestinal bacteria are controlled by one or a few nutritional substrates which a given strain can utilize most efficiently in the presence of H2S and at the prevailing conditions of pH and anaerobiosis. This hypothesis consequently implies that the populations of enterobacteria, such as the E. coli strain tested, and those of the predominant anaerobes are controlled by analogous mechanisms.

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Adhesion of vibrios to the small intestine may occur (i) by association of the bacteria with secreted mucus gel or (ii) by adherence of the bacteria to the surface of epithelial cells. In the present study, vibrios readily adhered to isolated brush border membranes obtained from rabbit intestinal epithelial cells. Adhesion was temperature dependent and required the presence of divalent cations such as calcium. The agglutination of human O erythrocytes by Vibrio cholerae was observed also, and the hemagglutination test appeared to detect the same mechanism that was involved in the adhesion of vibrios to brush borders. When the bacteria were grown in broth they were adhesive and hemagglutinating, but vibrios grown on agar plates or suspended in buffer for 15 min at 37 C lacked these abilities, even though they retained undiminished motility. These two model systems differed, however, in that strontium promoted only adhesion to brush borders. The significance of this difference remains to be determined. Vibrios were observed to penetrate intestinal mucus gel and occasionally to become entrapped in it. However, there was no evidence that vibrios attached to mucus gel.

Nonmotile vibrio mutants lacked the ability to adhere to rabbit intestinal brush border membranes and to agglutinate human group 0 erythrocytes, but motile revertant vibrios isolated from such strains expressed adhesiveness equivalent to that of the original parent. Two possible explanations for the relation between vibrio motility and adhesion in these assay systems are (i) that the rate of adhesion depends on thd rate of chance contact brought about by motility, and (ii) that the flagellum either acts as a carrier for the bacterial adhesin or may itself be the adhesin. The present study indicates, however, that the lack of adhesion by nonmotile vibrios did not depend on motility as such and did not involve greater rates of elution. Increasing the rate of contact between nonmotile vibrio mutants and brush border membranes by compaction did not restore the adhesive properties of the defective strains. Accordingly, we speculate that the flagellum may function in some indirect way that allows the expression of the adhesive properties, such as by acting as a carrier for a specific vibrio adhesin. Adhesion to brush borders and agglutination of human group 0 erythrocytes was specifically inhibited by L-fucose and various glycosides of Lr fucose and to a lesser extent by D-mannose. Vibrios adhered specifically to agarose beads that carried covalently linked L-fucose on their surfaces. The results suggest that L-fucose-containing structures of eukaryotic cell surfaces may function as receptors for the vibrio adhesin and may therefore be an important determinant of host susceptibility.

A method has been devised for inhibiting the normal enteric flora, permitting long term asymptomatic enteric infections of mice and guinea pigs with streptomycin-resistant strains of Shigella flexneri or Vibrio cholerae. Introduction of a streptomycin-resistant strain of E. coli into the intestinal tract of experimental animals resulted in a rapid elimination of the enteric pathogens studied. No in vitro production of antibiotic substances by this coli strain could be demonstrated. Active and oral passive immunization did not noticeably influence the number of Shigella or Vibrio organisms recoverable from the feces of infected animals.