Stenotrophomonas rhizophila IS26 reduces aflatoxin B1 by 86% on maize grains, linked to polyamine and oxidative stress genes.

Aflatoxin B1 (AFB1), a potent carcinogenic mycotoxin produced by Aspergillus flavus, poses a severe threat to global food and feed safety. Current strategies for managing AFB1 contamination often involve chemical treatments or physical removal, which can be costly, environmentally unfriendly, or impractical at scale. There is a critical need for sustainable and effective biocontrol agents to reduce AFB1 accumulation in agricultural commodities like maize. Endophytic bacteria, residing within plant tissues, offer a promising avenue for developing such biological solutions due to their natural association with plants and potential to interfere with fungal growth or toxin production.

Researchers investigated the biocontrol potential of the endophytic bacterium Stenotrophomonas rhizophila strain IS26 against toxigenic Aspergillus flavus NRRL 3251 in a maize grain assay. They compared the AFB1 inhibition efficacy of live IS26 cells against heat-killed cells and cell-free supernatant to differentiate contact-dependent from diffusible mechanisms. Phenotypic and biochemical characteristics, including protease, amylase, and esterase activities, were also assessed. Whole-genome sequencing of S. rhizophila strain IS26 was performed using Illumina reads, followed by annotation with NCBI-PGAAP and RAST to identify genes potentially involved in AFB1 reduction.

Live Stenotrophomonas rhizophila strain IS26 cells achieved a significant 86.30 ± 0.67% AFB1 inhibition on maize grains. This was markedly superior to the 22.82 ± 1.91% inhibition observed with heat-killed cells and 13.77 ± 5.72% with cell-free supernatant, indicating a predominant contact-dependent mechanism. Whole-genome sequencing revealed a 4.09 Mb genome with a 66.4% GC content, predicting 3,624 genes and 304 subsystems. Genome mining identified several gene clusters potentially contributing to AFB1 down-regulation or degradation. These included genes involved in polyamine biosynthesis pathways, oxidative stress response factors like alkyl hydroperoxide reductase, and the production of zinc-chelating peptides (cysteine- and histidine-rich).