The Role of Bacilli

Dr. Ankita Garkoti
May 5
7 min read

Updated: May 6

Unveiling the Agricultural Potential of Bacillus subtilis and Bacillus pumilus

Physical, chemical, and biological factors influence soil health and crop productivity. Among these, beneficial microbes play a crucial role—enhancing plant growth and development, boosting crop yield, and reducing reliance on synthetic fertilizers. A wide variety of bacterial species exhibit plant growth-promoting traits, with the genus Bacillus standing out as one of the most prominent due to its ability to produce a broad spectrum of enzymes, antibiotics, and bioactive metabolites that contribute significantly to agricultural productivity. (1).

Mode of Action of Bacillus

Role of Bacillus

Nurture Growth Biofertilizer

Conclusion

Bacillus subtilis and Bacillus pumilus are among the most effective plant growth-promoting bacteria and play a vital role in sustainable agriculture. These soil-dwelling, spore-forming bacteria colonize the rhizosphere (root zone) and significantly promote plant health. They enhance plant growth and suppress pathogens through multiple mechanisms, including enhancing soil nutrient availability, producing beneficial phytohormones and antimicrobial compounds, and activating induced systemic resistance (ISR) in plants.

Bacillus species not only promote plant growth and protect plants from pathogens, but they also engage in a mutualistic relationship with plants. The bacteria benefit from plant-derived nutrients and a favourable environment around the roots, while the plants benefit from enhanced growth and protection. This mutual relationship fosters the thriving of both the plant and the microbe.

Photo: Subtilis Harbors Multiple Plant Growth–Promoting Effects

This diagram shows that Bacillus subtilis harbors multiple plant growth–promoting effects, including improving nutrient availability, reducing abiotic stresses, and producing growth-promoting phytohormones. It also has biocontrol properties, for example, via producing antimicrobials, triggering induced systemic resistance, and competing with pathogens.

Mode of Action of Bacillus

Bacillus functions as a plant growth-promoting bacterium and an effective biocontrol agent, enhancing plant development while protecting against various pathogens. Bacillus species promote plant growth by producing phytohormones like auxins and gibberellins, enhancing root development and nutrient uptake. They solubilize phosphorus and mobilize other essential minerals, making them more accessible to plants. As biocontrol agents, Bacillus strains produce antibiotics, lytic enzymes, and siderophores that suppress harmful pathogens by breaking down their cell walls or starving them of nutrients. This dual action makes Bacillus highly effective in naturally improving crop health and productivity.

The rhizosphere is a region of soil surrounding plant roots influenced by root secretions and microbial activity; it serves as the primary site for nutrient uptake and hosts critical physiological, chemical, and biological processes essential for plant health (1). Various microorganisms, such as Bacillus species, inhabit the rhizosphere and play a vital role in promoting plant growth and enhancing overall plant health. Bacillus–plant interactions are mediated through complex chemical signaling involving diverse metabolites, enhancing plant growth and strengthening defense responses (1). The diagram illustrates the various compounds secreted by plant roots and bacteria and their chemical interactions within the rhizosphere.

Photo: Chemical Conversation between Plant and Bacillus

This diagram shows the molecules involved in Bacillus-plant interaction. Plants produce various compounds, including hormones, organic acids, flavonoids, and amino acids, as signaling molecules. Bacillus produces volatiles, lipopeptides such as surfactin, and phytohormones as signaling molecules.

Role of Bacillus

Bacillus subtilis and Bacillus pumilus both play important roles as plant-growth-promoting bacteria and biocontrol agents for plant pathogens.

1. Plant Growth-Promoting Bacteria:

Bacillus subtilis and Bacillus pumilus are classified as plant growth-promoting bacteria. While both belong to the Bacillus genus, phylogenetic analysis has distinguished Bacillus pumilus as a separate species from the Bacillus subtilis group (2). These bacteria are well-known for their ability to produce phytohormones and various plant growth-promoting substances, contributing significantly to enhanced root development and nutrient uptake. In addition to their beneficial effects on plant health, their capacity to form highly resilient spores makes them particularly valuable for agricultural applications. These spores are incredibly resistant to various abiotic stresses such as drought, temperature fluctuations, and nutrient limitations, ensuring the stability and effectiveness of bacterial formulations under field conditions.

A) Increases the nutrient availability for plants:

Soil is a dynamic, living system that naturally contains various essential nutrients. However, many of these nutrients are not in a form readily available for plant uptake. Beneficial microorganisms such as Bacillus subtilis and Bacillus pumilus play a crucial role in unlocking these nutrients. They help solubilize phosphorus and convert it into forms that plants can absorb more efficiently.

While Bacillus subtilis and Bacillus pumilus are not primary nitrogen-fixing bacteria like Rhizobium or Azotobacter, they can enhance nitrogen availability indirectly—by improving soil structure, stimulating microbial activity, or promoting plant root growth that supports nitrogen uptake. This contributes to enriching the soil with a vital nutrient that supports plant growth (2,3). Some microbes produce siderophores—low molecular weight, iron-binding protein molecules under iron-limited conditions. Many Bacillus spp. are known for their ability to produce siderophores, with Bacillus subtilis and Bacillus pumilus being two important examples. Siderophores can help suppress plant diseases and enhance plant growth by improving iron availability and limiting pathogen proliferation

These bacteria, Bacillus subtilis and Bacillus pumilus, act as effective plant growth promoters through various functions. They promote rhizobacteria, enhance nutrient availability and overall plant health, and contribute to sustainable agriculture.

B) Produces Plant Growth-Promoting Hormones

The ability of certain microbes to produce plant hormones is a key feature that makes them effective as biofertilizers, promoting root development and overall plant growth. Plant growth-promoting microorganisms synthesize phytohormones such as indole-3-acetic acid (IAA), gibberellins, and cytokinins, significantly influencing root development, cell division, and overall plant physiology, thereby contributing to improved crop productivity. Like many beneficial microbes, Bacillus subtilis and Bacillus pumilus can produce plant growth-promoting hormones (2,3,4). By influencing various physiological processes, these Bacillus species support vigorous plant growth and contribute to healthier and more productive crops, making them a valuable component of sustainable agricultural practices reducing the need for chemical inputs while promoting long-term soil fertility and crop performance.

C) Reduces Abiotic Stress

Abiotic stress caused by environmental factors can disrupt the normal physiological functions of plants and can significantly reduce crop yield and quality. Bacillus subtilis and Bacillus pumilus are well-known for mitigating abiotic stresses such as drought and salinity. Studies have shown that Bacillus pumilus can significantly promote plant growth under salinity stress (2). Bacillus subtilis has been shown to enhance plant tolerance to drought and salinity stress by improving water retention, nutrient uptake, and activating stress-responsive mechanisms (3). The bacteria achieve these effects by enhancing root growth, improving water and nutrient uptake, producing stress-related hormones, and activating the plant’s natural defense mechanisms. As a result, they help plants maintain growth and productivity under unfavourable environmental conditions.

2. Biocontrol for Plant Pathogens:

In addition to promoting plant growth, Bacillus species also serve as biocontrol agents, employing direct and indirect mechanisms to protect plants from pathogens by producing antimicrobial compounds and inducing systemic resistance. Bacillus subtilis and Bacillus pumilus act as effective biocontrol agents through multiple mechanisms. Plant protection by these species primarily occurs via two key processes:

A) Production of Antimicrobial Compounds

Bacillus species help protect plants by attacking harmful microbes in several ways. They can directly kill or stop the growth of plant pathogens by producing substances like antimicrobial compounds, including antibiotics, which effectively suppress various plant pathogens. Bacillus subtilis can reduce disease severity by directly inhibiting the growth of pathogens and reducing their virulence (6). This biocontrol capability reduces reliance on chemical pesticides, contributing to more sustainable and environmentally friendly agricultural practices.

B) Inducing Systemic Resistance

Bacillus species can induce systemic resistance (ISR) in plants, a defense mechanism that enhances the plant’s ability to resist a wide range of pathogens. In this process, the bacteria activate the plant immune system rapidly upon pathogen attack. When Bacillus colonizes the plant root zone, it secretes signaling molecules that activate its internal defense system. Induced systemic resistance triggered by Bacillus species is a sustainable and environmentally friendly way to protect crops, as it enhances the plant’s innate immunity without relying on chemical pesticides.

Photo: Biocontrol Mechanism of Bacillus

Nurture Growth Biofertilizer

Recognizing the importance of Bacillus species in agriculture, both as plant growth-promoting bacteria and as biocontrol agents, Nurture Growth Biofertilizer incorporates Bacillus subtilis, Bacillus pumilus and other beneficial microorganisms in its formulation. This powerful biofertilizer enhances crop yield by naturally supplying Phosphorus, Nitrogen and other essential nutrients, promoting robust plant growth and supporting sustainable agricultural practices. The beneficial phytohormones produced by Bacillus subtilis and Bacillus pumilus play a vital role in enhancing root development, shoot elongation, cell division, and overall plant physiology, ultimately leading to improved crop productivity. As a result, Nurture Growth Biofertilizer promotes vigorous plant growth, enhances soil fertility, and supports overall crop development.

The application of Nurture Growth Biofertilizer helps reduce dependence on environmentally harmful synthetic fertilizers. Beneficial microorganisms like Bacillus subtilis and Bacillus pumilus in the formulation break down insoluble phosphorus in the soil, converting it into forms that plants can readily absorb and enhancing plant growth while minimizing the need for chemical inputs. With Nurture Growth Biofertilizer, farmers can adopt more sustainable agricultural practices, maintain optimal crop yields, and improve soil health.

Conclusion

Plant microbes are vital in promoting plant growth and enhancing soil fertility. Bacillus species are significant in agriculture, as they function both as plant growth-promoting bacteria and as biocontrol agents that protect plants from harmful pathogens. Bacillus subtilis and Bacillus pumilus are two important species of the Bacillus genus, known for their dual role in promoting plant growth and providing plant protection. These beneficial microbes support plant development through nitrogen fixation, phosphorus solubilization, and providing other essential nutrients to plants. These microbes produce phytohormones that stimulate root and shoot growth and overall plant development. These microorganisms also protect plants by acting as biocontrol agents either by directly inhibiting plant pathogens by producing substances like antimicrobial compounds or by inducing systemic resistance within the plant.

Sustainable farming practices are essential for maintaining soil fertility, ensuring healthy crop production, and protecting the environment. Beneficial microbes such as Bacillus subtilis and Bacillus pumilus are notable tools for sustainable agriculture. They naturally enhance soil fertility, boost plant resistance to pathogens, and reduce the reliance on synthetic fertilizers.

Blogger Biography

Ankita is a Senior Science Officer at Nurture Growth Bio Fertilizer. She has over 9 years of experience in Agriculture research and extension. Her areas of specialization encompass Plant Pathology, Microbiology, Plant Protection, Organic farming, Biofertilizers, Organic Fertilizers, and Biocontrol agents.

She holds a Ph.D. in Plant Pathology and a master's degree in Botany with a specialization in Plant Pathology. Her doctoral research involved an in-depth study of lentil wilt and its management through various practices. She has extensive experience in organizing training programs on Organic and Natural farming techniques for farmers, agricultural growers, students and other stakeholders.