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Current IssueEditorial BoardOnline First ArticlesArchive
Current IssueEditorial BoardOnline First ArticlesArchive

ABSTRACT

Phytopathogenic bacteria (PPB) cause plant diseases, resulting in significant global economic losses. Over 200 bacterial species across 25 genera damage various plant species, predominantly from the Pseudomonadaceae, Xanthomonadaceae and Enterobacteriaceae families. These bacteria invade plant tissues and surfaces to live and obtain nourishment, often in collaboration with fungi and viruses, along with the impact of abiotic stress factors like chemical pollution, climate change, biodiversity loss, post-harvest loss, food waste and environmental degradation. These challenges collectively pose a substantial threat to global agricultural food production. Achieving global food security, sustainability in agriculture and enhancing the yield of high-quality healthy crops require effective control of plant bacterial diseases. Traditional chemical control methods have led to bacterial resistance, environmental pollution, high costs and negative impacts on human and ecological health. To address these issues, attention has shifted towards the alternatives of synthetic agrochemicals, with a focus on biopesticides. This paper comprehensively analyzes using Bacillus species as bioagents and investigating their mechanisms of action, including competition, antibiosis and induced plant resistance and evaluating their effectiveness in managing bacterial plant diseases. The findings suggest that Bacillus spp. offer a promising solution to sustainable agricultural practices, providing a safer and more eco-friendly approach to plant disease management.
 

INTRODUCTION

Plant pathogenic bacteria (PPB) cause diseases that result in significant economic losses, with crop damage estimated at 30-40%, amounting to between one billion dollars and five billion euros annually, particularly affecting pears, vineyards and apple orchards (Kannan et al., 2015; Khan et al., 2023; Mansfýeld  et al., 2012; Verhaegen et al., 2023). More than 200 bacterial species from 25 genera cause significant damage to diverse plant species (Buttimer et al., 2017). Most of these bacteria belong to the families Pseudomonadaceae, Xanthomonadaceae and Enterobacteriaceae, targeting both plant surface structures and internal tissues where they can obtain nutrients and establish habitats. These families include Acidovorax, Agrobacterium (some strains), Pectobacterium, Xanthomonas, Clavibacter, Burkholderia,  Pseudomonas (some strains), Streptomyces, Spiroplasma, Erwinia, Pantoea, Ralstonia and Xylella, which are a significant number of highly destructive phytopathogenic bacteria (Kannan et al., 2015; Sharma et al., 2023). The interaction of these bacteria with other pathogens such as fungi and viruses, alongside abiotic stressors like climate change, pollution and biodiversity loss, presents a substantial challenge to global agricultural food production (Martins et al., 2018). Therefore, maintaining global food security requires the application of management strategies to eradicate plant pathogenic bacteria by reducing their survival chances (Rizzo et al., 2021; Strange et al., 2005). Farmers often depend extensively on chemical fertilizers and pesticides (Satyadev Prajapati and Maurya, 2020). While chemical pesticides have proven effective in eradicating plant diseases in agriculture, the unfortunate consequence is that the misuse of certain chemicals, coupled with prolonged and excessive use, causes numerous environmental problems, the rapid emergence of resistance in phytopathogenic bacteria and pests and can be costly. Furthermore, when these chemicals accumulate in soil or plants, they can be harmful to humans and affect non-target animals, microbial communities and non-target plants (Said, 2023; Tudi et al., 2021). Consequently, scientists turned their attention to natural alternatives to artificial agrochemicals, leading to the improvement of biopesticides for controlling diseases and pests.
       
Biopesticides derive from various sources, including microorganisms, natural compounds and pesticidal substances with additional genetic material (Kumar et al., 2021). They offer safe, environmentally acceptable solutions for hazardous pesticides when used in agricultural and horticultural applications. This paper comprehensively analyzes the potential of Bacillus species as biocontrol agents, exploring their mechanisms of action and the advantages they provide in managing plant bacterial diseases.
 
Characteristics of the bacillus genus
 
Bacillus subtilis, the first identified Bacillus species, was discovered by Christian Gottfried Ehrenberg in 1835. He named the bacterium Vibrio subtilis because of its rod-shaped appearance (Jörg Stülke  et al., 2023; Sella et al., 2014). In 1872, Cohn renamed the bacterium to Bacillus subtilis (Harirchi et al., 2022). Since then, Bacillus has been discovered in a variety of environments, including ponds, air, soil, fresh and saline water and food as contaminants. It can also be present in the digestive tracts of animals like pigs (Haque et al., 2022; Wei et al., 2015). Bacillus species are rod-shaped, facultatively anaerobic, endospore-forming and catalase-positive bacteria with flagellar motility and variable sizes (0.5 to 10 μm) bacteria that grow best in a neutral pH (Márquez  et al., 2011; Radhakrishnan et al., 2017; Zhou et al., 2022). These species are frequently employed as bioagents for plant diseases because of their capability to form spores, which enable them to live in harsh environments. They release extracellular proteins, exhibit rapid growth and possess efficient biosynthetic pathways (Liu et al., 2020). Additionally, numerous organic and inorganic substances can be used by Bacillus species for growth and energy. Moreover, there are significant differences in nutritional needs among species. Certain strains, classified as prototrophs rely solely on an organic carbon source, while others, categorized as auxotrophs depend on the substrate for essential organic compounds (Logan NA and P, 2009). Certain Bacillus species, such as B. licheniformis, B. anthracis and B. cereus, can infect humans and animals, whereas B. thuringiensis is harmful to invertebrates (Pinos et al., 2021; Ramirez-Olea  et al., 2022).
 
Use of the Bacillus genus as a bioagent
 
Using Bacillus species for biological control is a practical, eco-friendly approach to managing plant diseases. Studies on employing Bacillus species as biological agents to manage plant diseases have been ongoing for a long time. Bacillus spp., particularly B. subtilis, are widely recognized as plant growth-promoting rhizobacteria that live in soil. They are crucial in giving plants resistance against abiotic and biotic stresses, as shown in Fig 1, through the formation of biofilms, induced systemic resistance and lipopeptide production. These species, at controlled concentrations, boost carbon sequestration, denitrify soils and aid in metal-contaminated soil bioremediation (Mahapatra et al., 2022). They release antimicrobial metabolites that can supplement or replace synthetic chemicals in biofertilizers and biopesticides to prevent plant diseases. They also produce endospores to survive adverse conditions, along with phytohormones, siderophores and the ability to solubilize potassium and phosphorus, enhancing plant growth and nutrient uptake (Collins and Jacobsen, 2003; Fan et al., 2019; Ongena et al., 2005).

Fig 1: Roles of Bacillus spp. in managing abiotic and biotic stresses in crops.


       
Many Bacillus species have antagonistic activity against a variety of phytopathogenic bacteria found in field crops including fruit trees, rice, corn and others (Li  et al., 2015).
       
The mode of action of Bacillus biological control for phytopathogenic bacteria is as follows:
 
Direct mode of action
 
Antibiosis
 
Antibiotics are small, low molecular-weight molecules produced mainly by soil-dwelling microbes as secondary metabolites. It is known that some Bacillus species, like B. amyloliquefaciens, B. mycoides, B. megaterium,  B. subtilis, B. cereus and B. licheniformis, can generate strong antibiotic substances that prevent phytopathogenic growth. These include both non-ribosomal and ribosomal cyclic peptides, which researchers have extensively studied (Cawoy et al., 2011; Li et al., 2015). A study by Bottone et al., (2003) stated that the Bacilli class produces about 167 different antibiotics. Of these, 23 are obtained from Bacillus subtilis and 66 from Bacillus brevis, with other Bacillus species contributing the remaining antibiotic peptides (Bottone and Peluso, 2003). Different Bacillus strains produce antimicrobial peptides that vary significantly and this variation impacts their effectiveness in inhibiting plant pathogenic bacteria (Basi-Chipalu  et al., 2022; Cladera-Olivera  et al., 2004). 
       
Bacillus species produce various bacteriocins with antimicrobial properties, including subtilin, thuricin, amylolysin, amylocyclicin, subtilosin A, subtilosin B and amysin (Abriouel et al., 2011). Some of these are trained in the biological management of phytopathogens. As an example, B. clausii GM17 produces Bac GM17, which is active in combating Agrobacterium tumefaciens, or B. thuringiensis subsp. kurstaki Bn1’s thuricin Bn1 inhibits Pseudomonas syringae and Pseudomonas savastanoi (Mouloud et al., 2013; Ugras et al., 2013). Furthermore, several antimicrobial substances, like bacillomycin D, zwittermicin A, fengycins, iturins and surfactins, are formed by Bacillus strains. These substances, which are referred to as cyclic lipopeptides (CLPs), have potent pathogen-inhibiting properties (Farace et al., 2014; Li et al., 2019). Several Bacillus variants, including Bacillus amyloliquefaciens A17 (currently B. velezensis) and B. amyloliquefaciens KPS46, have been shown to produce CLPs, with antibacterial activity (Chaisit Preecha  et al., 2010; Mora et al., 2011, 2015; Saha et al., 2016).
 
Siderophore production
 
Soil organisms frequently face competition for vital resources including nutrients, oxygen and space. Competition for space and nutrients around host plants between pathogenic and non-pathogenic microorganisms can result in biological control (Pal and McSpadden Gardener, 2006). The most common nutrient element over which beneficial microbes, such as Bacillus spp. and pathogenic microorganisms compete is Iron (Fe). There is intense competition because there is a shortage of bioavailable iron in the soil environment. The process responsible for this competition is known as Siderophore. Its primary function is to chelate iron, allowing it to be soluble and extracted from organic materials and minerals (Miljakovic et al., 2020). It is essential to biological control because it combats infectious agents for iron, limiting their accessibility (Beneduzi et al., 2012). A variety of bacteria, including Azospirillum, Azotobacter, Bacillus, Dickeya, Enterobacter, Kosakonia, Klebsiella, Methylobacterium, Nocardia, Pantoea, Paenibacillus, Pseudomonas, Serratia, Streptomyces and others, can produce siderophore.
       
Most studies documented the significance of siderophores in inhibiting plant bacterial diseases. In 2004, Kloepper et al., (1989) investigated the importance of siderophores in suppressing pathogens like Pectobacterium carotovorum, showcasing the potential of PGPR in managing plant diseases biologically (Kloepper et al., 2004). The effectiveness of Bacillus amyloliquefaciens strain S1 against C. michiganensis spp. Michiganensis, which causes canker disease in tomatoes, was reported. The effectiveness of strain S1 was linked to the synthesis of siderophores, antibacterial metabolites and lytic enzymes in vitro conditions (Gautam et al., 2019).
 
Production of hydrolytic and lytic enzymes
 
Bacillus Species are known as valuable reservoirs for a broad range of enzymes. B. licheniformis, B. subtilis and other microorganisms produce Keratinases which are widely used in animal feed, leather industries, detergent and feed, as well as in biomedical and healthcare applications (Gupta and Ramnani, 2006; Silva et al., 2014). The hydrolytic enzymes found in Bacillus species, such as glucanases, cellulases, lipases, proteases, chitinases and chitosanases, contribute to the effective degradation of essential elements of bacterial and fungal cell walls (Miljaković et al., 2020). Bacteria predominantly produce chitinases to break down chitin for energy consumption. Moreover, particular bacterial chitinases are utilized as potent bioagents for combating a range of plant diseases induced by fungal pathogens (Kumar et al., 2018; Saber et al., 2015). The breakdown of cell walls that results from interactions between Bacillus species and pathogens may be facilitated by the action of bacterial cellulases, lipases and proteases (Guleria et al., 2016). Previous research has been published on isolating these enzymes from Bacillus brevis, Bacillus subtilis, Bacillus halodurans, Bacillus circulans and Bacillus licheniformis (Planas, 2000).

Recent studies have extensively examined the production of lytic and hydrolytic enzymes by Bacillus species, emphasizing their promising role as biocontrol agents in managing plant diseases. The ability of B. amyloliquefaciens to form lytic enzymes like cellulase, protease, lipase and chitinase is associated with its potential to function as a bioagent against Clavibacter michiganensis ssp. Michiganensis (Gautam et al., 2019).
 
Indirect mode of action
 
Plant growth-promoting rhizobacteria (PGPR)
 
Plant growth-promoting rhizobacteria (PGPR) are beneficial living soil bacteria that colonize plant roots and promote plant growth (Kloepper et al., 1989). PGPRs improve plant growth directly by supplying compounds like phytohormones or aiding nutrient absorption and indirectly by limiting phytopathogens or inducing plant resistance (Glick, 1995). Some PGPRs form biofilms, which are structures made of bacteria encased in organic and inorganic compounds. Biofilms help PGPRs colonize plant roots, promoting a stable and sustained interaction with the host plant. Consequently, this may improve PGPR’s capacity to encourage plant development and offer pathogen protection (Haque et al., 2020; Karimi et al., 2022). Toxins such as bacillomycin, fengycin, iturin, surfactin and macrolactin, are released by Bacillus species when they form a biofilm on the root surface. By doing this activity, harmful bacterial populations are eliminated and the frequency of plant diseases is reduced (Elshakh et al., 2016; Huang et al., 2014).
       
The Bacillus genus, broadly studied and utilized in agriculture for its practical potential as PGPB, includes species like Priestia megaterium (formerly Bacillus megaterium), B. circulans, B. coagulans, B. subtilis, B. azotofixans, B. macerans and B. velezensis (Blake et al., 2021; Etesami et al., 2023; Fan et al., 2018).
       
Biocontrol agents derived from the Bacillus genus are frequently more potent compared to other PGPB species such as Pseudomonas spp., because These species have the superior capability to produce metabolites and form spores, thereby enhancing cell viability in commercially viable products (Haas and Défago, 2005). Three Bacillus strains isolated from the pepper rhizosphere in Turkey significantly reduced bacterial spot disease severity, achieving reductions ranging from 11% to 62% in field conditions and 38% to 67% in greenhouse experiments. Additionally, these strains enhanced plant growth and increased yield (Mirik et al., 2008).  
 
Induction of systemic resistance (ISR) and SAR
 
Plants have evolved defenses against abiotic and biotic stresses, including threats from herbivores, pests and phytopathogens. Their defense mechanisms, known as systemic acquired resistance (SAR), remain dormant until activated by pathogens, triggering a widespread response throughout the plant (Pandey et al., 2016; Pieterse et al., 2014). Two types of plant systemic resistance are: induced systemic resistance (ISR) and systemic acquired resistance (SAR). ISR refers to resistance activated by beneficial microbes and is typically independent of salicylic acid (SA), while, SAR is termed when induced resistance is initiated by pathogens and is closely associated with the salicylic acid signaling pathway (Pieterse et al., 2014; Yu et al., 2022). Beneficial microorganisms can activate defense mechanisms in the host through various pathways, providing plants resistance against several pathogens. B. cereus, B. atrophaeus, Bacillus amyloliquefaciens, Bacillus megaterium, Bacillus subtilis and similar microbes have been indicated to be effective against bacterial, viral and fungal invasions through ISR (Yu et al., 2022). Various studies investigated the impact of ISR-producing Bacillus spp. and SAR on phytopathogenic bacteria including Erwinia carotovora subsp. Carotovora, Pseudomonas syringae pv. tomato, Pectobacterium carotovorum, Xanthomonas citri subsp. Citri and Clavibacter michiganensis subsp. Michiganensis (Dadaşoğlu et al., 2020; Jang et al., 2022).

CONCLUSION

This review discusses the challenges posed by agrochemicals and explores viable alternatives, focusing on employing Bacillus species to biologically control plant-pathogenic bacteria (PPB). It also delves deeply into its mechanisms including antibiosis, the production of siderophores, lytic and hydrolytic, as well as ISR and SAR. Members of multiple Bacillus species, such as Bacillus subtilis, Bacillus licheniformis, Bacillus megaterium, Bacillus cereus, Bacillus mycoides, Bacillus amyloliquefaciens and Bacillus pumilus, are recognized as prolific producers of highly effective antibiotic molecules. These antibiotics, encompassing ribosomal (bacteriocins) and non-ribosomal cyclic peptides, inhibit the growth of phytopathogenic agents. Siderophore production is one of the main properties of Bacillus spp. enabling them to chelate iron and reduce its availability, thereby succeeding in competition with pathogens. Bacillus Species are recognized as valuable reservoirs for a wide range of enzymes. The hydrolytic enzymes found in Bacillus species, such as glucanases, cellulases, lipases, proteases, chitinases and chitosanases, may facilitate the effective degradation of key elements of bacterial and fungal cell walls. Bacillus spp. also create biofilm surrounding the root surface and releases toxic substances, including surfactin, macrolactin, bacillomycin, iturin and fengycin. This action results in the elimination of harmful bacterial populations and a decrease in disease incidence in plants. Bacillus amyloliquefaciens, B. atrophaeus, B. cereus, Bacillus megaterium, Bacillus subtilis and similar microbes have demonstrated efficacy against fungal, bacteria and viral invasions through induced systemic resistance. Finally, we recommend further in-depth research on beneficial bacteria such as Bacillus species and their genetic interactions with pathogens and plants to maintain biodiversity balance and mitigate the environmental challenges posed by pesticides.

ACKNOWLEDGEMENT

This paper was presented at the 8th International Student Science Congress held in Gaziantep, Turkey, from October 31 to November 3, 2024. I am deeply grateful to the conference organizers for granting me the opportunity to share my review.

CONFLICT OF INTEREST

The author confirms the absence of any conflicts of interest related to the publication of this paper.

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    ABSTRACT

    Phytopathogenic bacteria (PPB) cause plant diseases, resulting in significant global economic losses. Over 200 bacterial species across 25 genera damage various plant species, predominantly from the Pseudomonadaceae, Xanthomonadaceae and Enterobacteriaceae families. These bacteria invade plant tissues and surfaces to live and obtain nourishment, often in collaboration with fungi and viruses, along with the impact of abiotic stress factors like chemical pollution, climate change, biodiversity loss, post-harvest loss, food waste and environmental degradation. These challenges collectively pose a substantial threat to global agricultural food production. Achieving global food security, sustainability in agriculture and enhancing the yield of high-quality healthy crops require effective control of plant bacterial diseases. Traditional chemical control methods have led to bacterial resistance, environmental pollution, high costs and negative impacts on human and ecological health. To address these issues, attention has shifted towards the alternatives of synthetic agrochemicals, with a focus on biopesticides. This paper comprehensively analyzes using Bacillus species as bioagents and investigating their mechanisms of action, including competition, antibiosis and induced plant resistance and evaluating their effectiveness in managing bacterial plant diseases. The findings suggest that Bacillus spp. offer a promising solution to sustainable agricultural practices, providing a safer and more eco-friendly approach to plant disease management.
     

    INTRODUCTION

    Plant pathogenic bacteria (PPB) cause diseases that result in significant economic losses, with crop damage estimated at 30-40%, amounting to between one billion dollars and five billion euros annually, particularly affecting pears, vineyards and apple orchards (Kannan et al., 2015; Khan et al., 2023; Mansfýeld  et al., 2012; Verhaegen et al., 2023). More than 200 bacterial species from 25 genera cause significant damage to diverse plant species (Buttimer et al., 2017). Most of these bacteria belong to the families Pseudomonadaceae, Xanthomonadaceae and Enterobacteriaceae, targeting both plant surface structures and internal tissues where they can obtain nutrients and establish habitats. These families include Acidovorax, Agrobacterium (some strains), Pectobacterium, Xanthomonas, Clavibacter, Burkholderia,  Pseudomonas (some strains), Streptomyces, Spiroplasma, Erwinia, Pantoea, Ralstonia and Xylella, which are a significant number of highly destructive phytopathogenic bacteria (Kannan et al., 2015; Sharma et al., 2023). The interaction of these bacteria with other pathogens such as fungi and viruses, alongside abiotic stressors like climate change, pollution and biodiversity loss, presents a substantial challenge to global agricultural food production (Martins et al., 2018). Therefore, maintaining global food security requires the application of management strategies to eradicate plant pathogenic bacteria by reducing their survival chances (Rizzo et al., 2021; Strange et al., 2005). Farmers often depend extensively on chemical fertilizers and pesticides (Satyadev Prajapati and Maurya, 2020). While chemical pesticides have proven effective in eradicating plant diseases in agriculture, the unfortunate consequence is that the misuse of certain chemicals, coupled with prolonged and excessive use, causes numerous environmental problems, the rapid emergence of resistance in phytopathogenic bacteria and pests and can be costly. Furthermore, when these chemicals accumulate in soil or plants, they can be harmful to humans and affect non-target animals, microbial communities and non-target plants (Said, 2023; Tudi et al., 2021). Consequently, scientists turned their attention to natural alternatives to artificial agrochemicals, leading to the improvement of biopesticides for controlling diseases and pests.
           
    Biopesticides derive from various sources, including microorganisms, natural compounds and pesticidal substances with additional genetic material (Kumar et al., 2021). They offer safe, environmentally acceptable solutions for hazardous pesticides when used in agricultural and horticultural applications. This paper comprehensively analyzes the potential of Bacillus species as biocontrol agents, exploring their mechanisms of action and the advantages they provide in managing plant bacterial diseases.
     
    Characteristics of the bacillus genus
     
    Bacillus subtilis, the first identified Bacillus species, was discovered by Christian Gottfried Ehrenberg in 1835. He named the bacterium Vibrio subtilis because of its rod-shaped appearance (Jörg Stülke  et al., 2023; Sella et al., 2014). In 1872, Cohn renamed the bacterium to Bacillus subtilis (Harirchi et al., 2022). Since then, Bacillus has been discovered in a variety of environments, including ponds, air, soil, fresh and saline water and food as contaminants. It can also be present in the digestive tracts of animals like pigs (Haque et al., 2022; Wei et al., 2015). Bacillus species are rod-shaped, facultatively anaerobic, endospore-forming and catalase-positive bacteria with flagellar motility and variable sizes (0.5 to 10 μm) bacteria that grow best in a neutral pH (Márquez  et al., 2011; Radhakrishnan et al., 2017; Zhou et al., 2022). These species are frequently employed as bioagents for plant diseases because of their capability to form spores, which enable them to live in harsh environments. They release extracellular proteins, exhibit rapid growth and possess efficient biosynthetic pathways (Liu et al., 2020). Additionally, numerous organic and inorganic substances can be used by Bacillus species for growth and energy. Moreover, there are significant differences in nutritional needs among species. Certain strains, classified as prototrophs rely solely on an organic carbon source, while others, categorized as auxotrophs depend on the substrate for essential organic compounds (Logan NA and P, 2009). Certain Bacillus species, such as B. licheniformis, B. anthracis and B. cereus, can infect humans and animals, whereas B. thuringiensis is harmful to invertebrates (Pinos et al., 2021; Ramirez-Olea  et al., 2022).
     
    Use of the Bacillus genus as a bioagent
     
    Using Bacillus species for biological control is a practical, eco-friendly approach to managing plant diseases. Studies on employing Bacillus species as biological agents to manage plant diseases have been ongoing for a long time. Bacillus spp., particularly B. subtilis, are widely recognized as plant growth-promoting rhizobacteria that live in soil. They are crucial in giving plants resistance against abiotic and biotic stresses, as shown in Fig 1, through the formation of biofilms, induced systemic resistance and lipopeptide production. These species, at controlled concentrations, boost carbon sequestration, denitrify soils and aid in metal-contaminated soil bioremediation (Mahapatra et al., 2022). They release antimicrobial metabolites that can supplement or replace synthetic chemicals in biofertilizers and biopesticides to prevent plant diseases. They also produce endospores to survive adverse conditions, along with phytohormones, siderophores and the ability to solubilize potassium and phosphorus, enhancing plant growth and nutrient uptake (Collins and Jacobsen, 2003; Fan et al., 2019; Ongena et al., 2005).

    Fig 1: Roles of Bacillus spp. in managing abiotic and biotic stresses in crops.


           
    Many Bacillus species have antagonistic activity against a variety of phytopathogenic bacteria found in field crops including fruit trees, rice, corn and others (Li  et al., 2015).
           
    The mode of action of Bacillus biological control for phytopathogenic bacteria is as follows:
     
    Direct mode of action
     
    Antibiosis
     
    Antibiotics are small, low molecular-weight molecules produced mainly by soil-dwelling microbes as secondary metabolites. It is known that some Bacillus species, like B. amyloliquefaciens, B. mycoides, B. megaterium,  B. subtilis, B. cereus and B. licheniformis, can generate strong antibiotic substances that prevent phytopathogenic growth. These include both non-ribosomal and ribosomal cyclic peptides, which researchers have extensively studied (Cawoy et al., 2011; Li et al., 2015). A study by Bottone et al., (2003) stated that the Bacilli class produces about 167 different antibiotics. Of these, 23 are obtained from Bacillus subtilis and 66 from Bacillus brevis, with other Bacillus species contributing the remaining antibiotic peptides (Bottone and Peluso, 2003). Different Bacillus strains produce antimicrobial peptides that vary significantly and this variation impacts their effectiveness in inhibiting plant pathogenic bacteria (Basi-Chipalu  et al., 2022; Cladera-Olivera  et al., 2004). 
           
    Bacillus     species produce various bacteriocins with antimicrobial properties, including subtilin, thuricin, amylolysin, amylocyclicin, subtilosin A, subtilosin B and amysin (Abriouel et al., 2011). Some of these are trained in the biological management of phytopathogens. As an example, B. clausii GM17 produces Bac GM17, which is active in combating Agrobacterium tumefaciens, or B. thuringiensis subsp. kurstaki Bn1’s thuricin Bn1 inhibits Pseudomonas syringae and Pseudomonas savastanoi (Mouloud et al., 2013; Ugras et al., 2013). Furthermore, several antimicrobial substances, like bacillomycin D, zwittermicin A, fengycins, iturins and surfactins, are formed by Bacillus strains. These substances, which are referred to as cyclic lipopeptides (CLPs), have potent pathogen-inhibiting properties (Farace et al., 2014; Li et al., 2019). Several Bacillus variants, including Bacillus amyloliquefaciens A17 (currently B. velezensis) and B. amyloliquefaciens KPS46, have been shown to produce CLPs, with antibacterial activity (Chaisit Preecha  et al., 2010; Mora et al., 2011, 2015; Saha et al., 2016).
     
    Siderophore production
     
    Soil organisms frequently face competition for vital resources including nutrients, oxygen and space. Competition for space and nutrients around host plants between pathogenic and non-pathogenic microorganisms can result in biological control (Pal and McSpadden Gardener, 2006). The most common nutrient element over which beneficial microbes, such as Bacillus spp. and pathogenic microorganisms compete is Iron (Fe). There is intense competition because there is a shortage of bioavailable iron in the soil environment. The process responsible for this competition is known as Siderophore. Its primary function is to chelate iron, allowing it to be soluble and extracted from organic materials and minerals (Miljakovic et al., 2020). It is essential to biological control because it combats infectious agents for iron, limiting their accessibility (Beneduzi et al., 2012). A variety of bacteria, including Azospirillum, Azotobacter, Bacillus, Dickeya, Enterobacter, Kosakonia, Klebsiella, Methylobacterium, Nocardia, Pantoea, Paenibacillus, Pseudomonas, Serratia, Streptomyces and others, can produce siderophore.
           
    Most studies documented the significance of siderophores in inhibiting plant bacterial diseases. In 2004, Kloepper et al., (1989) investigated the importance of siderophores in suppressing pathogens like Pectobacterium carotovorum, showcasing the potential of PGPR in managing plant diseases biologically (Kloepper et al., 2004). The effectiveness of Bacillus amyloliquefaciens strain S1 against C. michiganensis spp. Michiganensis, which causes canker disease in tomatoes, was reported. The effectiveness of strain S1 was linked to the synthesis of siderophores, antibacterial metabolites and lytic enzymes in vitro conditions (Gautam et al., 2019).
     
    Production of hydrolytic and lytic enzymes
     
    Bacillus Species are known as valuable reservoirs for a broad range of enzymes. B. licheniformis, B. subtilis and other microorganisms produce Keratinases which are widely used in animal feed, leather industries, detergent and feed, as well as in biomedical and healthcare applications (Gupta and Ramnani, 2006; Silva et al., 2014). The hydrolytic enzymes found in Bacillus species, such as glucanases, cellulases, lipases, proteases, chitinases and chitosanases, contribute to the effective degradation of essential elements of bacterial and fungal cell walls (Miljaković et al., 2020). Bacteria predominantly produce chitinases to break down chitin for energy consumption. Moreover, particular bacterial chitinases are utilized as potent bioagents for combating a range of plant diseases induced by fungal pathogens (Kumar et al., 2018; Saber et al., 2015). The breakdown of cell walls that results from interactions between Bacillus species and pathogens may be facilitated by the action of bacterial cellulases, lipases and proteases (Guleria et al., 2016). Previous research has been published on isolating these enzymes from Bacillus brevis, Bacillus subtilis, Bacillus halodurans, Bacillus circulans and Bacillus licheniformis (Planas, 2000).

    Recent studies have extensively examined the production of lytic and hydrolytic enzymes by Bacillus species, emphasizing their promising role as biocontrol agents in managing plant diseases. The ability of B. amyloliquefaciens to form lytic enzymes like cellulase, protease, lipase and chitinase is associated with its potential to function as a bioagent against Clavibacter michiganensis ssp. Michiganensis (Gautam et al., 2019).
     
    Indirect mode of action
     
    Plant growth-promoting rhizobacteria (PGPR)
     
    Plant growth-promoting rhizobacteria (PGPR) are beneficial living soil bacteria that colonize plant roots and promote plant growth (Kloepper et al., 1989). PGPRs improve plant growth directly by supplying compounds like phytohormones or aiding nutrient absorption and indirectly by limiting phytopathogens or inducing plant resistance (Glick, 1995). Some PGPRs form biofilms, which are structures made of bacteria encased in organic and inorganic compounds. Biofilms help PGPRs colonize plant roots, promoting a stable and sustained interaction with the host plant. Consequently, this may improve PGPR’s capacity to encourage plant development and offer pathogen protection (Haque et al., 2020; Karimi et al., 2022). Toxins such as bacillomycin, fengycin, iturin, surfactin and macrolactin, are released by Bacillus species when they form a biofilm on the root surface. By doing this activity, harmful bacterial populations are eliminated and the frequency of plant diseases is reduced (Elshakh et al., 2016; Huang et al., 2014).
           
    The Bacillus genus, broadly studied and utilized in agriculture for its practical potential as PGPB, includes species like Priestia megaterium (formerly Bacillus megaterium), B. circulans, B. coagulans, B. subtilis, B. azotofixans, B. macerans and B. velezensis (Blake et al., 2021; Etesami et al., 2023; Fan et al., 2018).
           
    Biocontrol agents derived from the Bacillus genus are frequently more potent compared to other PGPB species such as Pseudomonas spp., because These species have the superior capability to produce metabolites and form spores, thereby enhancing cell viability in commercially viable products (Haas and Défago, 2005). Three Bacillus strains isolated from the pepper rhizosphere in Turkey significantly reduced bacterial spot disease severity, achieving reductions ranging from 11% to 62% in field conditions and 38% to 67% in greenhouse experiments. Additionally, these strains enhanced plant growth and increased yield (Mirik et al., 2008).  
     
    Induction of systemic resistance (ISR) and SAR
     
    Plants have evolved defenses against abiotic and biotic stresses, including threats from herbivores, pests and phytopathogens. Their defense mechanisms, known as systemic acquired resistance (SAR), remain dormant until activated by pathogens, triggering a widespread response throughout the plant (Pandey et al., 2016; Pieterse et al., 2014). Two types of plant systemic resistance are: induced systemic resistance (ISR) and systemic acquired resistance (SAR). ISR refers to resistance activated by beneficial microbes and is typically independent of salicylic acid (SA), while, SAR is termed when induced resistance is initiated by pathogens and is closely associated with the salicylic acid signaling pathway (Pieterse et al., 2014; Yu et al., 2022). Beneficial microorganisms can activate defense mechanisms in the host through various pathways, providing plants resistance against several pathogens. B. cereus, B. atrophaeus, Bacillus amyloliquefaciens, Bacillus megaterium, Bacillus subtilis and similar microbes have been indicated to be effective against bacterial, viral and fungal invasions through ISR (Yu et al., 2022). Various studies investigated the impact of ISR-producing Bacillus spp. and SAR on phytopathogenic bacteria including Erwinia carotovora subsp. Carotovora, Pseudomonas syringae pv. tomato, Pectobacterium carotovorum, Xanthomonas citri subsp. Citri and Clavibacter michiganensis subsp. Michiganensis (Dadaşoğlu et al., 2020; Jang et al., 2022).

    CONCLUSION

    This review discusses the challenges posed by agrochemicals and explores viable alternatives, focusing on employing Bacillus species to biologically control plant-pathogenic bacteria (PPB). It also delves deeply into its mechanisms including antibiosis, the production of siderophores, lytic and hydrolytic, as well as ISR and SAR. Members of multiple Bacillus species, such as Bacillus subtilis, Bacillus licheniformis, Bacillus megaterium, Bacillus cereus, Bacillus mycoides, Bacillus amyloliquefaciens and Bacillus pumilus, are recognized as prolific producers of highly effective antibiotic molecules. These antibiotics, encompassing ribosomal (bacteriocins) and non-ribosomal cyclic peptides, inhibit the growth of phytopathogenic agents. Siderophore production is one of the main properties of Bacillus spp. enabling them to chelate iron and reduce its availability, thereby succeeding in competition with pathogens. Bacillus Species are recognized as valuable reservoirs for a wide range of enzymes. The hydrolytic enzymes found in Bacillus species, such as glucanases, cellulases, lipases, proteases, chitinases and chitosanases, may facilitate the effective degradation of key elements of bacterial and fungal cell walls. Bacillus spp. also create biofilm surrounding the root surface and releases toxic substances, including surfactin, macrolactin, bacillomycin, iturin and fengycin. This action results in the elimination of harmful bacterial populations and a decrease in disease incidence in plants. Bacillus amyloliquefaciens, B. atrophaeus, B. cereus, Bacillus megaterium, Bacillus subtilis and similar microbes have demonstrated efficacy against fungal, bacteria and viral invasions through induced systemic resistance. Finally, we recommend further in-depth research on beneficial bacteria such as Bacillus species and their genetic interactions with pathogens and plants to maintain biodiversity balance and mitigate the environmental challenges posed by pesticides.

    ACKNOWLEDGEMENT

    This paper was presented at the 8th International Student Science Congress held in Gaziantep, Turkey, from October 31 to November 3, 2024. I am deeply grateful to the conference organizers for granting me the opportunity to share my review.

    CONFLICT OF INTEREST

    The author confirms the absence of any conflicts of interest related to the publication of this paper.

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