Submitted to: Microbial Ecology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/17/2006
Publication Date: 6/10/2006
Citation: Palumbo, J.D., Baker, J.L., Mahoney, N.E. 2006. Isolation of bacterial antagonists of aspergillus flavus from almonds. Microbial Ecology. DOI:10.1007/s00248-006-9096-y. URL http://dx.doi.org/10.1007/s00248-006-9096-y. Interpretive Summary: Bacterial populations from almond samples were tested to find individual bacteria that could inhibit the growth of Aspergillus flavus, a toxin-producing fungus. Using tests on agar medium and in liquid medium, 171 bacterial isolates were identified that could either reduce fungal growth or inhibit the production of the toxin. Those bacteria were also tested for their ability to degrade components of fungal cell walls, as a mechanism for their antifungal activity. Following identification of the bacteria using genetic and biochemical tests, a set of 20 bacterial strains were selected to study further. These 20 strains were grown with the toxin-producing A. flavus and the fungal mass was measured to determine how well the bacteria inhibited the fungal growth. These experiments show the potential use of bacteria, isolated from almonds, to be re-applied to almonds and used to control fungal growth and toxin production.
Technical Abstract: Bacteria were isolated from California almond orchard samples to evaluate their potential antifungal activity against aflatoxin-producing Aspergillus flavus. Fungal populations from the same samples were examined to determine the incidence of aflatoxigenic Aspergillus species. Antagonistic activities of the isolated bacterial strains were screened against a non-aflatoxigenic nor mutant of A. flavus, which accumulates the pigmented aflatoxin precursor, norsolorinic acid (NOR) under conditions conducive to aflatoxin production. Using solid and liquid media in coculture assays, 171 bacteria isolated from almond flowers, immature nut fruits and mature nut fruits showed inhibition of A. flavus growth and/or inhibition of NOR accumulation. Bacterial isolates were further characterized for production of extracellular enzymes capable of hydrolyzing chitin or yeast cell walls. Molecular and physiological identification of the bacterial strains indicated that the predominant genera isolated were Bacillus, Pseudomonas, Ralstonia, and Burkholderia, as well as several plant-associated enteric and non-enteric bacteria. A set of 20 isolates was selected for further study based on their species identification, antifungal phenotypes and extracellular enzyme production. Quantitative assays using these isolates in liquid coculture with a wild-type, aflatoxin-producing A. flavus strain showed that a number of strains completely inhibited fungal growth in three different media. These results indicate the potential for development of bacterial antagonists as biological control agents against aflatoxigenic aspergilli on almonds.