Aflatoxins are potent carcinogenic and toxic substances that are produced primarily by Aspergillus flavus and Aspergillus parasiticus. We found that a bacterium remarkably inhibited production of norsolorinic acid, a precursor of aflatoxin, by A. parasiticus. This bacterium was identified as Achromobacter xylosoxidans based on its 16S ribosomal DNA sequence and was designated A. xylosoxidans NFRI-A1. A. xylosoxidans strains commonly showed similar inhibition. The inhibitory substance(s) was excreted into the medium and was stable after heat, acid, or alkaline treatment. Although the bacterium appeared to produce several inhibitory substances, we finally succeeded in purifying a major inhibitory substance from the culture medium using Diaion HP20 column chromatography, thin-layer chromatography, and high-performance liquid chromatography. The purified inhibitory substance was identified as cyclo(L-leucyl-L-prolyl) based on physicochemical methods. The 50% inhibitory concentration for aflatoxin production by A. parasiticus SYS-4 ؍( NRRL2999) was 0.20 mg ml Aflatoxins are highly toxic, carcinogenic, and teratogenic secondary metabolites that are produced by certain strains of Aspergillus flavus and Aspergillus parasiticus (reviewed in reference 20). Recently, several strains of Aspergillus nomius, Aspergillus pseudotamarii, Aspergillus bombycis, and Aspergillus ochraceoroseus have also been reported to produce aflatoxin. These fungi are ubiquitous and grow on a variety of agricultural products under appropriate temperature and moisture conditions. Aflatoxins have been detected in numerous agricultural commodities, such as cereal grains, whole wheat, rye breads, oil seeds, cottonseed, etc. (25). The toxicity and carcinogenicity of aflatoxins have made contaminated commodities a significant health hazard all over the world. In fact, the incidence of liver cancer is high in regions with high endemic aflatoxin concentrations. Furthermore, the annual costs resulting from crop losses due to aflatoxin contamination and the costs involved in monitoring and disposal of contaminated commodities affect the agricultural economy (34).Many studies have focused on developing aflatoxin control strategies, including genetic engineering for crop resistance, biological control with competitive, nonaflatoxigenic strains of the fungus A. flavus (6, 9), and regulation of aflatoxin biosynthesis by fungicides, pesticides, inhibitory substances originating from plants, and microbial substances (15,24,27). However, most of these strategies have been shown to be limited in effectiveness. Many microorganisms have been studied to control aflatoxin production. Sweedy and Dobson (32) have reported that bacteria, yeast, molds, actinomycetes, and algae can be used to lower aflatoxin levels in foods and feeds. Ono et al. have reported that aflastatin A, which has been isolated from mycelial extracts of Streptomyces sp., effectively inhibits aflatoxin production (17, 23). Saprophytic yeasts, such as Pichia anomala, Candida krusei, and others, have...
