The study, published this week in Applied and Environmental Microbiology, an open-access journal of the American Society for Microbiology, was led by microbiologist Eric Déziel of the Institut national de la recherche scientifique (INRS) in Quebec, Canada. Déziel describes C. michiganensis as a genuinely problematic pathogen, particularly in greenhouse settings where dense planting, shared equipment, and constant human handling create ideal conditions for transmission. Once the bacterium gains a foothold in a facility, it typically establishes itself in one section before workers inadvertently carry it from plant to plant as they prune, tie, and harvest. The result is a creeping wave of infection that is notoriously difficult to contain with conventional tools.

What makes the new research distinctive is its dual focus. Most studies of biological control agents, Déziel notes, tend to concentrate on a single objective: either suppressing a pathogen or stimulating plant growth. The Canadian team set out to find microbes capable of doing both at once. Their screening strategy was deliberately broad and systematic, drawing on a resource the laboratory had been building for roughly two decades. For about twenty years, researchers in Déziel’s group have pursued biological solutions to agricultural problems, accumulating a large collection of bacterial strains isolated from a wide variety of food plants and environments.

The opportunity to apply that collection to tomato canker arose a few years ago, during the COVID-19 pandemic, when tomato growers brought the problem of bacterial canker in greenhouses to the lab’s attention. Déziel admits he had never heard of the disease at the time, but the growers’ concerns were enough to launch an intensive search for answers. The team began at home, so to speak, testing some 500 strains from their own collection for the ability to antagonize C. michiganensis in laboratory assays. These strains had been isolated from strawberries, lettuce, apples, and other foods, and had been recovered from leaves, roots, and other plant parts. Crucially, none of them had previously been isolated from bacteria-infected plants or fruits, meaning the researchers were working with microbes drawn from healthy contexts rather than from diseased tissue.

The initial screen was designed to cast a wide net. In antagonism assays, candidate bacteria are grown alongside the pathogen, allowing researchers to observe whether the candidate produces compounds, competes for resources, or otherwise interferes with the pathogen’s growth. From the 500 strains tested, a few dozen showed promising activity against C. michiganensis. That shortlist then faced a second, more demanding filter: the strains had to do more than fight the pathogen. They also needed to promote plant growth, a property that matters enormously for any biocontrol agent intended for real-world agriculture, since growers are far more likely to adopt a treatment that delivers a visible agronomic benefit in addition to disease protection.

The evaluation proceeded in stages, moving from simple bacterial cultures to living plants. This progression is essential in biocontrol research because activity observed on a laboratory plate does not always translate to the complex environment of a plant’s surfaces and internal tissues. When the team tested their shortlisted strains on tomato plants infected with bacterial canker, the experiments ultimately yielded three bacterial strains that demonstrated a clear dual benefit. One of these, Pantoea agglomerans SO16PY, originally isolated from the leaf of a healthy tomato plant, proved especially interesting: it delayed the onset of canker symptoms in infected plants by up to a week and reduced the overall severity of the disease. The other two strains, both members of the species Pseudomonas marginalis, similarly restricted both the development and the severity of the disease in infected plants.

The significance of a week’s delay should not be underestimated in commercial greenhouse production. Bacterial canker moves quickly through a crop, and every day of slowed progression gives growers additional time to detect the outbreak, remove affected plants, and adjust handling practices to limit the spread that occurs through workers’ hands and tools. Reduced disease severity, meanwhile, translates directly into preserved yield and plant vigor. Combined with the growth-promoting effects observed in the same strains, these microbes appear to function as what researchers sometimes describe as plant probiotics: living allies that shift the balance of the plant’s microbial environment away from pathogens and toward health.

The practical context for this work is particularly important for organic growers. Déziel points out that bacterial canker can be a significant problem for organic farmers, who have fewer chemical options available to them. Although organic producers can purchase seeds that are certified or guaranteed to be free of the pathogen, the bacterium may still find its way into a greenhouse through contaminated equipment, infected transplants, or residual inoculum in the facility. Once inside, the disease follows its familiar pattern of localized establishment followed by mechanical spread through routine work. Today’s treatment solutions are limited, Déziel adds, and the few commercial options that do exist show poor efficacy, leaving growers with largely preventive measures and little recourse once infection takes hold.

That gap between need and available tools is precisely where the three beneficial strains could eventually make a difference. The researchers emphasize, however, that considerable development work remains before these microbes could be harnessed for commercial use. The central challenge is formulation and delivery: the bacteria must be stabilized in a product form that maintains their viability and effectiveness over time. As Déziel explains, the team needs to find a way to stabilize the bacteria so that their shelf life is long enough for the product to be actually usable in a greenhouse. A biocontrol agent that loses potency within days of manufacture is of little value to a grower who needs to apply it on a schedule determined by the crop and the disease, not by the laboratory.

If that stabilization challenge can be solved, the payoff could be substantial. The three strains identified in this study represent a sustainable, environmentally friendly approach to improving plant health in agriculture, one that works with the natural antagonisms among microbes rather than against them. For an industry grappling with a seedborne pathogen that spreads easily and resists conventional treatment, bacteria that can both hold the disease at bay and help the crop grow stronger offer a genuinely new kind of tool. The work also underscores the value of long-term microbial collections and of listening to growers: a problem brought to the lab’s attention during a pandemic has now produced candidate biocontrol agents isolated from healthy tomato leaves and other food plants, strains that had been sitting in a freezer for years waiting for the right question to be asked of them.

Subject of Research: Identification of beneficial bacterial strains that suppress tomato bacterial canker and promote tomato plant growth

Article Title: Beneficial bacteria support tomato growth & suppress disease

Article References: Beneficial bacteria support tomato growth & suppress disease. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: tomato, bacterial canker, Clavibacter michiganensis, biological control, Pantoea agglomerans, Pseudomonas marginalis, plant growth promotion, greenhouse agriculture, seedborne disease, microbiome, organic farming, Applied and Environmental Microbiology

The significance of the findings begins with the crop itself. Mung bean, originally domesticated in India, is now cultivated on roughly 7.3 million hectares globally, producing an estimated 6 million tons annually, with India, Myanmar, China, and Indonesia dominating the market. As a legume, it fixes atmospheric nitrogen through symbiosis with rhizobia, improving soil fertility and providing nitrogen inputs for subsequent crops, which makes it especially valuable in resource-limited farming systems. In Ethiopia, mung bean is an emerging crop concentrated in the North Shewa and South Wollo zones of the Amhara region and parts of Benishangul-Gumuz. Yet productivity remains stubbornly low: traditional varieties average about 0.5 megagrams per hectare, improved varieties can exceed 1.5 megagrams under optimal conditions, and the national average of 0.9 megagrams per hectare sits roughly 20 percent below the world average. Unpredictable rainfall, fungal pathogens, pests, poor agronomic practices, and limited access to quality inputs all conspire to hold yields back.

That combination of biotic and abiotic pressures is precisely where endophytic bacteria enter the picture. Endophytes are microorganisms that live within plant tissues without causing disease, and they can benefit their hosts in two broad ways. Direct growth promotion occurs through the production of phytohormones, improved nutrient mobilization, and the induction of systemic tolerance to environmental stress. Indirect protection comes through competitive exclusion of pathogens and the secretion of antimicrobial metabolites such as lipopeptides, polyketides, and volatile organic compounds. Fungal pathogens like Aspergillus flavus pose a particularly serious threat because they produce aflatoxins that contaminate seeds and endanger human and animal health, making biocontrol agents that can target this fungus a priority for both food security and food safety.

To find candidate microbes, the team grew fifty Shewa-type mung bean seeds in vertisol soil at Addis Ababa Science and Technology University without any chemical fertilizers, irrigating weekly over three months until the plants reached the fruiting stage. They then collected fifty healthy root systems and fifty seeds and subjected them to a rigorous surface sterilization protocol involving sequential ethanol and sodium hypochlorite treatments. Crucially, the researchers validated the sterilization using two complementary methods: imprinting sterilized tissues onto nutrient agar plates and spread-plating the final rinse water. Only samples showing zero visible microbial growth after overnight incubation were used, ensuring that the bacteria recovered genuinely lived inside the plant tissues rather than on their surfaces. Macerated tissues were serially diluted and spread across four different growth media to capture as broad a range of the culturable endophytic community as possible.

The isolation effort yielded twenty-eight distinct bacterial endophytes, with more recovered from seeds, sixteen, than from roots, twelve, a distribution suggesting that seed-associated microbiomes may facilitate vertical transmission of beneficial bacteria across generations. Eight representative isolates, selected for their morphological diversity and labeled GMB R1, R2, S1 through S6, were chosen for detailed characterization. Biochemical profiling revealed that three isolates were Gram-positive while five were Gram-negative, all produced catalase, and all fermented glucose. Stress tolerance screening then uncovered a striking spectrum of resilience. Four isolates, GMB R1, R2, S5, and S6, showed robust thermotolerance, maintaining optical densities above 0.58 at 45 degrees Celsius, while others were thermosensitive and grew poorly above 37 degrees. GMB R2 displayed exceptional pH flexibility, losing less than 15 percent growth across the full range from pH 4 to pH 10, and GMB S5 proved the strongest halotolerant strain, sustaining growth at sodium chloride concentrations as high as 12 percent.

Heavy metal tolerance added another dimension to the functional portrait. When grown in media amended with lead acetate at concentrations from 50 to 300 micrograms per milliliter, all isolates grew at the baseline level, but growth inhibition increased with concentration in a strain-dependent manner. GMB R1 stood out as the most lead-tolerant, maintaining strong optical density across the entire gradient, with GMB R2 and GMB S5 close behind. The authors suggest that mechanisms such as exopolysaccharide production, metal sequestration, and efflux systems likely underpin this tolerance, and they highlight the strains as candidates for colonizing plants in heavy metal-contaminated agricultural soils, a growing problem in many intensively farmed regions.

The biocontrol results may prove the most immediately compelling. In dual culture assays on malt extract agar, GMB R1 and GMB R2 significantly suppressed Aspergillus flavus, reducing fungal mycelial growth by 66.7 percent and 73.3 percent respectively, compared with control colony diameters of 4.5 millimeters after seven days. Enzymatic profiling revealed strain-specific specialization that could contribute to this antagonism: GMB R1 showed the highest cellulase activity with an enzymatic index of 2.5, GMB R2 the highest chitinase activity at 1.6, and GMB S5 the highest protease activity at 2.0. Chitinase is particularly relevant to antifungal action because chitin is a structural component of fungal cell walls. Siderophore production, which starves pathogens of iron, was detected in seven of the eight isolates, and six isolates produced ACC deaminase, an enzyme that lowers plant ethylene levels and helps seedlings withstand stress. Notably, the study found no trade-off between stress adaptation and biocontrol capacity, meaning the hardiest strains were also among the best fungal antagonists.

Molecular identification using 16S rRNA gene sequencing confirmed the taxonomy of three key isolates: GMB R1 matched Pseudomonas fluorescens with 99.87 percent similarity, GMB R2 showed 100 percent identity with Pantoea agglomerans, and GMB S5 aligned fully with Serratia marcescens. Sequencing failed for the remaining five isolates, likely due to suboptimal DNA template quality, so those strains are currently identified only by morphological and biochemical profiles, a limitation the authors candidly acknowledge. Partial 16S rRNA sequences for the three characterized strains have been deposited in NCBI GenBank under accession numbers OR974898.1, OR974899.1, and OR974900.1, and phylogenetic analysis using the maximum-likelihood method with 1,000 bootstrap replicates supported their taxonomic placements.

Perhaps the most eye-catching numbers come from the plant growth assays, conducted not on mung bean itself but on wheat (Triticum aestivum) and rapeseed (Brassica napus), demonstrating cross-host efficacy. In germination tests, GMB S6 increased wheat seedling radicle length to 12.4 centimeters and plumule extension to 18.9 centimeters, producing a vigor index of 3,067.4, roughly 140 percent above the uninoculated control. In pot trials over 90 days, GMB S5 produced wheat roots 77 percent longer than controls with an 85 percent increase in root dry weight. On rapeseed, GMB R1 extended radicle length by 149 percent and achieved a vigor index of 2,114.6, an 82 percent improvement, while GMB R2 generated the longest plumules at 9.1 centimeters and boosted rapeseed plumule length by 127 percent. Germination rates stayed high across all treatments, between 94 and 97 percent, indicating that the bacteria enhanced post-germinative development rather than germination itself. One isolate, GMB S2, actually inhibited plumule growth, a reminder that endophyte effects are strain- and context-dependent and that not every endophyte is benign or beneficial.

The authors are careful to frame the work as an exploratory baseline rather than a field-ready prescription. The sampling came from a single site and a single cultivar, physiological assays relied on optical density rather than viable cell counts, heavy metal testing used only lead as a model contaminant, and the inoculated bacteria were not re-isolated from wheat and rapeseed tissues to confirm true endophytic establishment. Field conditions, with competition from indigenous microbiota and environmental variability, will likely produce more modest gains than the laboratory and greenhouse results suggest. Still, the combination of thermotolerance, halotolerance, heavy metal resistance, siderophore production, lytic enzymes, ACC deaminase activity, antifungal activity against aflatoxigenic fungi, and cross-host growth promotion makes GMB R1, GMB R2, and GMB S5 stand out as promising multifunctional bioinoculants. Future work, including whole-genome sequencing, testing on the native mung bean host, consortium design, and multi-location randomized field trials, will determine whether these Ethiopian endophytes can translate their laboratory performance into real yields for marginal agroecosystems, where the need for sustainable alternatives to chemical inputs has never been greater.

Subject of Research: Stress-tolerant endophytic bacteria isolated from Ethiopian mung bean with biocontrol and plant growth-promoting traits

Article Title: Stress-resilient endophytic bacteria from mung bean (Vigna radiata L.) with biocontrol and plant growth-promoting potential

Article References: Abatenh, E., Kebede, M., & Abda, E. M. (2026). Stress-resilient endophytic bacteria from mung bean (Vigna radiata L.) with biocontrol and plant growth-promoting potential. International Microbiology. https://doi.org/10.1007/s10123-026-00897-y

Image Credits: AI Generated

DOI: 10.1007/s10123-026-00897-y

Keywords: endophytic bacteria, mung bean, Vigna radiata, biocontrol, Aspergillus flavus, plant growth promotion, Pseudomonas fluorescens, Pantoea agglomerans, Serratia marcescens, abiotic stress tolerance, aflatoxin, sustainable agriculture