. As few as five of the species of bacteria commonly found in human faeces–, , , and —when grown together in anaerobic continuous flow cultures exerted antagonistic effects on as great as those given by mixed bacteria from extracts of human faeces. In a single culture, the population of was c. 10 cfu/ml but in mixed cultures with the five antagonistic bacteria or mixed faecal bacteria it was reduced to c. 10 cfu/ml. Antagonism appeared to be the result of competition for the growth limiting amino acids, arginine, serine, threonine and aspartic acid. Optimal manifestation of antagonism required the presence of carbon sources fermentable only by antagonistic bacteria, such as lactose 0.1%, w/v, sucrose 0.1% (w/v) and starch 0.2-0.3% w/v. These carbohydrates promoted the growth of the antagonistic bacteria, particularly and . However, an increase in concentration by several fold of any one of four growth-limiting amino acids in the medium diminished the antagonistic effects and the population of rose 10-10-fold.


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