Long-Term Fungicide Exposure Makes Foodborne Pathogen Bacillus cereus More Lethal

Agrarian landscapes across the globe are saturated with pesticides, and more than 60 percent of the world’s agricultural land is now considered at risk of pesticide contamination. A new laboratory study published in Current Research in Food Science suggests that this constant chemical pressure may be quietly reshaping one of the most common foodborne pathogens. Researchers report that when the spore-forming bacterium Bacillus cereus is repeatedly exposed to certain fungicides over a month, the pathogen becomes tougher to kill with antibiotics, forms more biofilm, and—most strikingly—becomes significantly more lethal to its infection host, all without a single detectable mutation in its genome.

The findings come from a team led by Hsin-Yu Wang, Chun Ming How, Yong-Shan Li, Yuqing Mao, Thanh H. Nguyen and Chia-Cheng Wei, who set out to answer a question that has become increasingly urgent in food safety research: do non-antibiotic chemicals, particularly the fungicides sprayed widely on fruit and vegetable crops, push bacteria toward resistance or heightened virulence? Prior work has hinted at the danger. Azoxystrobin and carbendazim have been shown to enrich bacterial resistomes in nematode guts, tebuconazole can promote the spread of multidrug-resistant plasmids in soil bacteria, and chlorothalonil facilitates metabolic adaptation in soil microbial communities. But whether such exposure produces phenotypic resistance and increased pathogenicity in a major foodborne pathogen remained largely untested.

Bacillus cereus was an obvious candidate for scrutiny. The Gram-positive, spore-forming organism is found in 36 to 45 percent of dairy products, vegetables, beans and cereals, and it is capable of causing food poisoning, eye infections, anthrax-like progressive pneumonia, fulminant sepsis and central nervous system infections. Multidrug-resistant strains of the species have already emerged in hospital wastewater, and its versatility—including the ability to build biofilms of varied architecture—makes any shift in its behavior a serious public health concern.

The researchers first screened eight widely used fungicides against B. cereus: chlorothalonil (CHT), propineb (PRO), tebuconazole (TEB), azoxystrobin, propiconazole, mancozeb, carbendazim and triadimefon. Three of them—CHT at 8 micromolar, TEB at 500 micromolar and PRO at 175 micromolar—completely inhibited bacterial growth within 24 hours and were selected for long-term adaptation experiments. The design was demanding: every day for 30 days, the bacteria endured a three-hour fungicide challenge followed by recovery and regrowth in fresh medium. Survival trajectories differed by compound. Under chlorothalonil, survival dipped to about 70 percent on day one but rebounded within 24 hours. Tebuconazole initially halved the population before recovery stabilized around day eight. Propineb proved the harshest pressure, dropping survival below five percent on day six before the bacteria clawed back to stable levels by day twelve. The bacterium, in short, adapted to all three chemical regimes.

Whole-genome sequencing of the adapted lineages delivered a surprising verdict: no meaningful genetic mutations. Phylogenetic comparison against reference strains and variant-calling analyses found the treated bacteria essentially identical to their ancestors. Instead of classical, mutation-driven resistance, the adaptation appears to be physiological—a reversible, non-heritable tolerance state akin to the persister-cell and stress-response phenomena documented in bacteria subjected to repeated antibiotic cycles. Similar patterns have been reported when Listeria monocytogenes and uropathogenic Escherichia coli were exposed to disinfectants such as benzalkonium chloride and triclosan, with minimum inhibitory concentrations rising without stable genetic change.

The phenotypic consequences, however, were substantial. Biofilm formation—often a shield against both immune attack and antimicrobial agents—was initially suppressed during early exposure but rose significantly in tebuconazole-adapted bacteria from day ten onward and climbed markedly in propineb-exposed cells by day ten. Statistical testing confirmed significant effects of the fungicide treatment, the duration of exposure, and their interaction on biofilm output. Antibiotic challenge assays revealed a parallel erosion of susceptibility. Bacteria adapted to chlorothalonil grew significantly better than controls in gentamicin at 4, 6 and 8 micrograms per milliliter; tebuconazole- and propineb-adapted lineages also outgrew controls at key gentamicin doses, and chlorothalonil- and tebuconazole-adapted cells showed improved growth at 8 micrograms per milliliter of tetracycline. Because no growth occurred at concentrations of 16 micrograms per milliliter or above, the strains do not meet formal clinical criteria for resistance—but the shift toward tolerance was clear and reproducible.

The most dramatic result emerged in living hosts. Using the nematode Caenorhabditis elegans, a genetically tractable infection model whose intestinal epithelium provides a biologically meaningful readout of colonization and killing, the team measured how fungicide-adapted bacteria fared against unadapted controls. All three adapted lineages killed worms significantly faster than the parent strain, with log-rank tests showing p values below 0.001. Tebuconazole-adapted bacteria were especially aggressive: worm survival collapsed within two days, and by day three most of the animals were dead. Follow-up colonization assays showed that tebuconazole-adapted B. cereus also established significantly higher intestinal loads in the worms, indicating that the fungicide had promoted persistence within the host gut, not merely faster killing.

Transcriptional profiling of the tebuconazole-adapted lineage offers a mechanistic window into these changes. Quantitative real-time PCR revealed significantly elevated expression of genes encoding the non-hemolytic enterotoxin (nheC) and the hemolysin BL complex (hblA, hblC and hblD)—toxins that disrupt intestinal epithelial cells—alongside upregulation of purC and purL, which support purine biosynthesis and extracellular DNA release during early biofilm formation, and calY, a bifunctional matrix protein that promotes adhesion to host tissues. The efflux-pump gene smr was also induced, a plausible explanation for the reduced antibiotic susceptibility, and one that echoes efflux upregulation seen in stressed Mycobacterium tuberculosis. Importantly, the elevated virulence and resistance gene expression persisted even when the adapted bacteria were subsequently exposed to gentamicin, suggesting that the stress-adapted state complicates antibiotic treatment rather than simply surviving it.

The authors are careful to frame the work as hazard identification rather than a direct portrait of what happens on farms or in food. The experiments used a single reference strain, BCRC15850, and the exposure concentrations—particularly 500 micromolar tebuconazole and 175 micromolar propineb—exceed the residue levels typically reported on treated foods, although the chlorothalonil dose is of the same order of magnitude as residues found in some food commodities. Local bioavailable concentrations in soil and produce depend on moisture, adsorption, formulation and degradation, so the laboratory model of recurrent acute stress cannot be directly translated into field-level risk estimates. Nor should the transcriptional findings be generalized beyond the tebuconazole lineage without confirming that chlorothalonil- and propineb-adapted bacteria share the same regulatory program. Nonetheless, the study reveals an understudied scenario in which persistent sublethal chemical stress can harden a major foodborne pathogen—improving its resilience, deepening its virulence and weakening the grip of frontline antibiotics—without any mutational fingerprint. Whether such phenotypes persist after fungicide withdrawal, and whether they arise in the genetically diverse field isolates that actually contaminate the food supply, are the questions the team now hopes will drive the next round of research.

Subject of Research: Effects of long-term fungicide exposure on adaptation, antibiotic tolerance and virulence of the foodborne pathogen Bacillus cereus

Article Title: Long-term fungicide exposure promotes bacterial adaptation and increases virulence of foodborne pathogen Bacillus cereus in Caenorhabditis elegans

Article References: Wang, H.-Y., How, C. M., Li, Y.-S., Mao, Y., Nguyen, T. H., & Wei, C.-C. (2026). Long-term fungicide exposure promotes bacterial adaptation and increases virulence of foodborne pathogen Bacillus cereus in Caenorhabditis elegans. Current Research in Food Science, 13, Article 101572. https://doi.org/10.1016/j.crfs.2026.101572

Image Credits: AI Generated

DOI: 10.1016/j.crfs.2026.101572

Keywords: Bacillus cereus, fungicide exposure, antimicrobial tolerance, biofilm formation, Caenorhabditis elegans, tebuconazole, chlorothalonil, propineb, virulence genes, food safety, foodborne pathogen, efflux pumps