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).
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).
