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

Several experiments were employed to test the role of competition in the ability of an atoxigenic strain of Aspergillus flavus to inhibit the aflatoxin contamination of developing cotton bells. In initial tests, nitrate-nonutilizing mutants were used to follow seed infection by toxigenic and atoxigenic strains of A. flavus in coinoculated bells. Competitive exclusion was found to contribute to the effect of the atoxigenic strain on contamination, but results suggested a second mechanism may also have been in effect. Aflatoxin contamination by the toxigenic strain was similarly inhibited by an atoxigenic strain in vivo and in liquid fermentation, and the atoxigenic strain was equally effective when applied at spore concentrations either equal to those of the toxigenic strain or one-half those of the toxigenic strain. The atoxigenic strain reduced aflatoxin production in vitro when mycelial balls of the two strains were mixed after a 48-h fermentation period, which suggested that close intertwining of mycelia was not required and that aflatoxin biosynthesis could be interrupted even after initiation. The atoxigenic strain did not degrade aflatoxins in vitro, and both culture 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 both by physically excluding the toxigenic strain during infection and by competing for nutrients required for aflatoxin biosynthesis.