COMPETITIVE-EXCLUSION OF A TOXIGENIC STRAIN OF ASPERGILLUS-FLAVUS BY AN ATOXIGENIC STRAIN

Several experiments were employed to test the role of competition in t he ability of an atoxigenic strain of Aspergillus flavus to inhibit th e aflatoxin contamination of developing cotton bells. In initial tests , nitrate-nonutilizing mutants were used to follow seed infection by t oxigenic and atoxigenic strains of A. flavus in coinoculated bells. Co mpetitive exclusion was found to contribute to the effect of the atoxi genic strain on contamination, but results suggested a second mechanis m may also have been in effect. Aflatoxin contamination by the toxigen ic strain was similarly inhibited by an atoxigenic strain in vivo and in liquid fermentation, and the atoxigenic strain was equally effectiv e when applied at spore concentrations either equal to those of the to xigenic strain or one-half those of the toxigenic strain. The atoxigen ic strain reduced aflatoxin production in vitro when mycelial balls of the two strains were mixed after a 48-h fermentation period, which su ggested that close intertwining of mycelia was not required and that a flatoxin biosynthesis could be interrupted even after initiation. The atoxigenic strain did not degrade aflatoxins in vitro, and both cultur e filtrates and mycelial extracts of the atoxigenic strain stimulated aflatoxin production by the toxigenic strain. The results suggest that the atoxigenic strain may interfere with the contamination process bo th by physically excluding the toxigenic strain during infection and b y competing for nutrients required for aflatoxin biosynthesis.