Beneficial soil bacteria show promise in suppressing dangerous food toxins

A research team from the Université de Toulouse has revealed that certain soil-dwelling bacteria from the genus Streptomyces can inhibit both the growth and toxin production of Aspergillus flavus and Aspergillus parasiticus — two fungi notorious for producing aflatoxins, among the most potent natural carcinogens known.

Published in Microbiology (2015), the study demonstrates that Streptomyces strains can repress key genes involved in aflatoxin biosynthesis, reducing toxin levels to undetectable amounts in some cases. The finding adds a powerful natural tool to the growing arsenal of biological control strategies against mycotoxin contamination in food and feed.

Aflatoxins: Hidden killers in global food supplies

Aflatoxins are toxic compounds produced by A. flavus and A. parasiticus when crops like maize, peanuts, and tree nuts are stored in warm, humid conditions. Chronic exposure has been linked to liver cancer, immune system impairment, and stunted growth in children. Many countries now regulate aflatoxin levels in food and feed, but chemical and physical decontamination methods remain expensive and limited.

Microbial teamwork: How Streptomyces inhibits Aspergillus

In controlled lab experiments, six Streptomyces strains were grown alongside A. flavus and A. parasiticus. The bacterial-fungal interactions were closely monitored for growth suppression and toxin production.

Results showed that Streptomyces griseoplanus (strain S35) and S. roseolus (strain S06) achieved the greatest reductions, cutting aflatoxin B1 and B2 levels by up to 100%, while also inhibiting fungal growth.

Interestingly, for A. flavus, toxin reduction occurred even without a corresponding decrease in fungal biomass — suggesting that Streptomyces acts at a molecular level rather than simply competing for space or nutrients.

Gene-level insights: Repressing the aflatoxin factory

Using reverse-transcriptase quantitative PCR (RT-qPCR), the researchers measured the expression of five genes central to aflatoxin biosynthesis:

aflD, aflM, aflP — structural genes involved in toxin synthesis
aflR and aflS — regulatory genes controlling toxin production

They found that Streptomyces significantly repressed aflM and aflS expression, both of which are crucial for aflatoxin B1 formation.
For instance, S. roseolus (S06) reduced aflM expression by up to 100-fold in A. parasiticus, corresponding with a complete absence of detectable toxins.

A promising step toward safer food systems

The study identifies Streptomyces griseoplanus S35 as a leading biocontrol candidate for future agricultural use. Because Streptomyces are common, non-toxic soil bacteria, they can be safely deployed as bioinoculants or seed coatings to protect crops during growth and storage.

Unlike fungicides, these bacteria act naturally and target the toxin-producing genes directly, rather than killing the fungus outright—an advantage that may limit resistance development.

Looking ahead

The researchers plan to extend their work to field-level trials and examine the metabolic compounds responsible for this repression effect. Understanding these molecular interactions could enable the development of microbial-based products to prevent aflatoxin accumulation across global food chains.