Impact of a Streptomyces (AS1) strain and its metabolites on control of Aspergillus flavus and aflatoxin B1 contamination in vitro and in stored peanuts

Abstract

Six actinomycetes were isolated from peanuts in Egypt. Of these, a Streptomyces strain (AS1) was found in in vitro assays to inhibit directly or via secondary metabolites both germination and growth of Aspergillus flavus. Tests of the AS1 cells for direct control of A. flavus populations or aflatoxin B1 (AFB1) production on stored peanuts was unsuccessful over 14-day storage periods. However, crude extracts of AS1 metabolites at 50 and 100 ppm completely inhibited spore germination of conidia of A. flavus in vitro over 48 h. Comparison of solvents for extracting the metabolites showed that the ethyl acetate extract was most effective. This gave greater than 85% inhibition of mycelial growth at these concentrations at different water availabilities (water activity; a w; 0.95, 0.92, and 0.89) and 25°C. Doses of 50, 200, and 500 ppm of AS1 metabolites significantly inhibited populations of A. flavus on stored peanuts at two water stress levels (0.90, 0.93 a w) at 25°C over 14-day storage periods. The amounts of AFB1 produced by A. flavus on peanuts stored at 0.90 a w were significantly decreased by AS1 metabolites for only 7 days. However, at 0.93 a w doses of 200 and 500 ppm significantly controlled AFB1 accumulation in peanuts for 14 days.