Bacillus subtilis- An Overview and Applications

Bacillus subtilis is a Gram-positive, rod-shaped bacterium that forms heat-resistant, dormant spores . It is commonly found in the soil and can also colonize the gastrointestinal tracts of animals and humans. It is not pathogenic and is considered "Generally Regarded as Safe" (GRAS) by the Food and Drug Administration.

Bacillus subtilis was one of the first bacteria to be studied by microbiologists. It was originally named Vibrio subtilis by Christian Gottfried Ehrenberg in 1835, and renamed Bacillus subtilis by Ferdinand Cohn in 1872. It has a single circular chromosome with about 4.2 million base pairs and 4100 protein-coding genes.

Bacillus subtilis is a model organism for studies of sporulation, gene regulation, cell differentiation, and biofilm formation. It has a complex life cycle that involves different developmental stages and responses to environmental signals. It can also exchange genetic material with other bacteria through transformation, transduction, and conjugation.

Bacillus subtilis produces various enzymes, antibiotics, vitamins, and other compounds that have industrial and biotechnological applications. It is used for the production of detergents, food additives, pharmaceuticals, biopesticides, and biopolymers. It can also degrade various pollutants and toxic substances in the environment.

Bacillus subtilis is a versatile and adaptable bacterium that can survive in extreme conditions and interact with other organisms. It is an important contributor to the soil ecosystem and a valuable resource for biotechnology.

Bacillus subtilis is a Gram-positive, rod-shaped, endospore-forming bacterium that belongs to the genus Bacillus and the family Bacillaceae. The genus Bacillus comprises more than 100 different species that are grouped into manageable and better-defined groups based on phylogenetic analysis of 16S rRNA gene sequences and DNA-DNA hybridization.

The initial classification of Bacillus species was based on phenotypic, cultural, and metabolic characteristics of the bacteria. However, these criteria were not sufficient to resolve the complex relationships among the species and to distinguish closely related species. Therefore, molecular methods like 16S rRNA gene sequencing and DNA-DNA hybridization were employed to group the species based on their genetic similarity and evolutionary history.

B. subtilis belongs to group 1 of Bacillus, which is also known as the B. subtilis group or the B. cereus group. This group consists of different species that are industrially important for the production of different compounds or are pathogenic to humans and animals. Some of the closely related species to B. subtilis in this group are B. licheniformis, B. cereus, B. anthracis, and B. thuringiensis.

B. subtilis is further classified into two subspecies; B. subtilis subsp. subtilis and B. subtilis subsp. spizizenii. These subspecies cannot be distinguished on the basis of phenotypic characteristics and require genotypic analysis like multilocus sequence typing (MLST) or whole-genome sequencing (WGS). The subspecies differ in their geographical distribution, ecological niche, and genetic diversity. B. subtilis subsp. subtilis is more widespread and diverse than B. subtilis subsp. spizizenii, which is mainly found in North America and Europe.

The following is the taxonomical classification of B. subtilis:

• Domain: Bacteria
• Phylum: Firmicutes
• Class: Bacilli
• Order: Bacillales
• Family: Bacillaceae
• Genus: Bacillus
• Species: Bacillus subtilis
• Subspecies: Bacillus subtilis subsp. subtilis and Bacillus subtilis subsp. spizizenii

Bacillus subtilis is a ubiquitous bacterium that can be found in various habitats throughout the world, ranging from soil to water to animal and human body surfaces.

• The most important and natural habitat of B. subtilis is soil of different kinds, ranging from acid to alkaline, cold to hot, and fertile to desert. The types of strains living in the habitats depend on the water content and deposits.
• B. subtilis can survive in a wide range of different temperatures from 15°C to 55°C. Some variety of B. subtilis can also tolerate high salinity and pH levels.
• The availability of B. subtilis in different environments is due to the distribution of bacterial spores in the form of aerosols. These spores are resistant to physical and chemical agents like temperature, disinfectants, antibiotics, and toxic compounds.
• Bacterial spores are also found in high concentrations in dried foods like spices, milk powder, and other products. These spores are dispersed easily by the wind, which allows the spores to migrate to long distances and discover new ecological niches.
• B. subtilis are heterotrophic organisms that are isolated from environments with complex nutrient availability and environmental conditions.
• The occurrence of B. subtilis in soil and the rhizospheric area might exist in a close relationship with the plants by helping in the production of phytohormones and enhancement of root nodulation.
• Some spores of B. subtilis can also be found in animal surfaces like the human intestinal tract and skin surfaces and can be isolated from samples like human feces.
• B. subtilis can also be found in fresh water, coastal waters, and oceans. However, the presence of B. subtilis in aquatic environments might be due to spore dispersal rather than active growth.

Bacillus subtilis is a rod-shaped, Gram-positive bacterium that can form a tough, protective endospore, allowing it to tolerate extreme environmental conditions. A bacterial rod is a symmetrical cylinder with rounded ends. A significant difference in pressure across the cytoplasmic membrane pushes the cell wall into a specific shape. The cell wall of B. subtilis is composed of a thick peptidoglycan layer called murein, which gives rigidity and strength to the cell. The peptidoglycan layer can be stained by crystal violet, resulting in a purple color on Gram stain.

The cells of B. subtilis are usually single or in pairs, but rarely form chains. The cells are motile with peritrichous flagella, which are hair-like structures that surround the cell and help it move through liquids. The cells have a single circular chromosome that contains about 4000 protein-coding genes. The chromosome is located in the cytoplasm, along with other structures like ribosomes, plasmids, and granules. During cell division, filament-forming proteins run along the longer axis of the cell and push the original and replicated DNA to each end.

Bacillus subtilis can transform into spores when exposed to stress, such as nutrient deprivation, heat, or disinfectants. Spores are dormant forms of bacteria that have a thick coat and a reduced metabolism. Spores can survive for long periods in harsh environments and germinate when conditions become favorable again. Spores are visible inside the cell by spore staining, which uses malachite green and safranin as dyes. Spores are usually located centrally or paracentrally in the cell and have an ellipsoidal or cylindrical shape.

The following image shows the morphology of Bacillus subtilis cells and spores:

• The colony morphologies of B. subtilis are highly variable, within and between strains, which may give the appearance of a mixed culture during growth on an artificial medium .
• In spite of the diversity, the colonies of Bacillus species can be recognized on agar plates quite easily.
• The growth on a simple medium like nutrient agar might result in the swarming growth of the bacteria through the plate. This can be avoided by increasing the agar content of the media.
• Bacillus species usually have simple nutrient requirements which allow their growth in simple non-selective media like Nutrient Agar.
• The optimum temperature for the growth of B. subtilis is 28-30°C with a minimum temperature of 5-20°C and a maximum temperature of 45-55°C.
• Growth of B. subtilis can be seen within the pH range of 5.5 to 8.5, but the growth of some strains might be limited even within the said range.
• The growth of B. subtilis can occur on a minimal medium with glucose and ammonium salt as the sole sources of carbon and nitrogen, respectively.
• Most of the strains can tolerate 7% NaCl in the medium, but some can tolerate up to 10% NaCl.
• Even though B. subtilis are known as obligate aerobes; some restricted growth can be observed under anaerobic conditions in complex media with glucose or even nitrate.
• In liquid culture, LB broth is commonly used for the culture of B. subtilis. The growth is observed in the form of turbidity, and the cells begin to settle down as the growth ceases.

The following are some cultural characteristics of B. subtilis in different culture media:

Bacillus subtilis in Nutrient Agar

• The colonies of B. subtilis on nutrient agar are round to irregular in shape. The isolates obtained from soil samples tend to form swarming growth throughout the plate .
• The size of the colonies is also variable ranging between 2-3 mm in diameter as the younger cultures tend to be larger and older colonies shrink up in size.
• The colonies have varying margins varying from undulate to fimbriate. The colonies are opaque with surfaces that are dull or even wrinkled.
• The color of the colonies is mostly white but can range between creamy and brown. Some strains produce varying pigments like creamy, yellow, orange, pink, and red to brown and black depending on the source or sample.
• These pigments are often observed in potato agar or glucose-containing agar medium.
• Strains that produce brown or black pigments were formerly called Bacillus subtilis var aterrimus, whereas those producing brownish-black pigment on tyrosine-containing media were name B. subtilis var niger.

Bacillus subtilis in Blood Agar

• B. subtilis form grey or white-colored colonies that are round, opaque, flat, and dry on blood agar supplemented with 5% rabbit blood .
• The colonies are medium-sized (ranging between 3-4 mm in diameter) that often dry on the surface as the culture dries out .
• Most strains of B. subtilis show β-hemolysis in the form of clearing of the media with the hemolysis of red blood cells. This is more common in B. subtilis obtained from clinical samples than from environmental samples .

Bacillus subtilis in Tryptic Soy Agar

• White to creamy colored colonies of B. subtilis are obtained on Tryptic Soy Agar. The colonies are circular or irregular in shape depending on the strain and the conditions for growth.
• The colonies have an irregular margin, and they are mostly flat. The surface is opaque and mucoid.
• On this agar, optimal growth occurs at 35°C under aerobic conditions. Some species may be facultatively anaerobic and might grow better in some 2% CO2.
  
  
  

  Biochemical Characteristics of Bacillus subtilis

Bacillus subtilis is a versatile bacterium that can utilize various substrates and produce different enzymes and metabolites. The biochemical characteristics of B. subtilis can be determined by performing different tests that measure the fermentation of carbohydrates, the production of gas, the hydrolysis of proteins and other compounds, and the enzymatic reactions of the bacterium. The following table summarizes some of the common biochemical tests and their results for B. subtilis:

Bacillus subtilis can also ferment different carbohydrates and produce acid or acid and gas as end products. The fermentation of carbohydrates can be detected by using phenol red broth with different sugars added to it, such as glucose, lactose, sucrose, mannitol, etc. The production of acid lowers the pH of the medium and turns the indicator yellow, while the production of gas forms bubbles in a Durham tube inverted inside the broth tube.

The following table shows some of the fermentation results of B. subtilis for different carbohydrates:

Rhamnose Negative (-ve) Ribose Positive (+ve) Salicin Positive (+ve) Sorbitol Positive (+ve) Starch Positive (+ve) Sucrose Positive (+ve) Trehalose Positive (+ve) Xylose Positive (+ve)

Bacillus subtilis can also produce different enzymes that hydrolyze various compounds such as proteins, lipids, nucleic acids, etc.

The following table shows some of the enzymatic reactions of B. subtilis:

Enzyme Test Result Explanation Arginine Dehydrolase Negative (-ve) B. subtilis does not produce arginine dehydrolase enzyme that converts arginine to citrulline and ammonia. Casein Hydrolysis Positive (+ve) B. subtilis produces caseinase enzyme that breaks down casein protein in milk agar to amino acids. Esculin Hydrolysis Positive (+ve) B.subtilis produces esculinase enzyme that splits esculin into esculetin and glucose. Lecithinase Negative (-ve) B.subtilis does not produce lecithinase enzyme that hydrolyzes lecithin to phosphatidylcholine and diglyceride. Lysine Negative (-ve) B.subtilis does not produce lysine decarboxylase enzyme that converts lysine to cadaverine and carbon dioxide. Ornithine Decarboxylase Negative (-ve) B.subtilis does not produce ornithine decarboxylase enzyme that converts ornithine to putrescine and carbon dioxide. Phenylalanine Deaminase Negative (-ve) B.subtilis does not produce phenylalanine deaminase enzyme that removes an amino group from phenylalanine. Tyrosine Hydrolysis Negative (-ve) B.subtilis does not produce tyrosine hydroxylase enzyme that adds a hydroxyl group to tyrosine.

Virulence Factors of Bacillus subtilis

Bacillus subtilis is a ubiquitous bacterium that is generally regarded as safe (GRAS) by the Food and Drug Administration. It is widely used as a model organism for cell differentiation and biotechnology, as well as a probiotic and biocontrol agent in agriculture and food industries. However, some strains of B. subtilis have been associated with opportunistic infections in humans and animals, as well as food poisoning and spoilage. The virulence factors of B. subtilis are not well understood, but some possible mechanisms have been suggested based on experimental studies and genomic analysis.

Toxin production

One of the possible virulence factors of B. subtilis is the production of toxins that can cause damage to host cells or tissues, or elicit immune responses. B. subtilis produces several extracellular enzymes that have proteolytic, lipolytic, or hemolytic activities, such as lecithinase, subtilisin, phospholipase C, and sphingomyelinase. These enzymes can degrade host cell membranes, release cellular contents, or interfere with signal transduction pathways. Some of these enzymes have been implicated in food poisoning outbreaks caused by B. subtilis-contaminated foods.

B. subtilis also produces a lipopeptide antibiotic called surfactin, which has antimicrobial, antiviral, antitumoral, and hypocholesterolemic properties. Surfactin can disrupt the integrity of bacterial and eukaryotic membranes by forming pores or micelles. Surfactin can also modulate the immune system by inducing cytokine production, activating macrophages, and stimulating natural killer cells. However, surfactin can also cause adverse effects in some individuals, such as allergic reactions, inflammation, or cytotoxicity.

Adhesion and invasion

Another possible virulence factor of B. subtilis is the ability to adhere to and invade host cells or tissues, which can facilitate colonization, dissemination, and evasion of host defenses. B. subtilis has been shown to bind to various host receptors, such as fibronectin, collagen, laminin, vitronectin, and integrins. These receptors are involved in cell adhesion, migration, differentiation, and signaling. B. subtilis can also produce extracellular polysaccharides or capsules that can mediate adhesion or protect the bacteria from phagocytosis or complement-mediated killing.

B. subtilis can also invade host cells by different mechanisms, such as endocytosis, membrane ruffling, or actin polymerization. These mechanisms can allow B. subtilis to enter epithelial cells, endothelial cells, macrophages, or leukocytes. Once inside the host cells, B. subtilis can survive and replicate by escaping from the phagosomes or lysosomes, or by modulating the intracellular environment. B. subtilis can also induce apoptosis or necrosis of host cells by activating caspases or releasing toxins.

Spore formation

A third possible virulence factor of B. subtilis is the formation of endospores that can resist harsh environmental conditions and persist in the host for long periods of time. B. subtilis forms spores when exposed to nutrient limitation or stress signals. The spores are composed of a core containing DNA and ribosomes surrounded by a cortex layer and a proteinaceous coat. The spores are highly resistant to heat, desiccation, radiation, chemicals, antibiotics, and disinfectants. The spores can also germinate when they encounter favorable conditions or specific germinants.

The spore formation of B. subtilis can contribute to its virulence by allowing the bacteria to survive in unfavorable environments or during host immune responses. The spores can also be dispersed by air currents or water droplets and reach new hosts or niches. The spores can also trigger inflammatory responses in the host by activating pattern recognition receptors or complement system. The spores can also cause diseases such as anthrax or tetanus when they germinate and produce toxins in the host tissues.

Conclusion

Bacillus subtilis is a versatile bacterium that has both beneficial and harmful effects on humans and animals. The virulence factors of B. subtilis are not well characterized but may involve toxin production, adhesion and invasion of host cells or tissues, and spore formation. These factors may enable B. subtilis to cause opportunistic infections or food poisoning in some cases. However, most strains of B. subtilis are harmless or even beneficial for human health and industrial applications.

Role of Bacillus subtilis in animal and plant diseases

Bacillus subtilis is a Gram-positive, rod-shaped bacterium that is widely distributed in various environments, such as soil, water, air, and animal and human body surfaces. It is generally regarded as a safe and beneficial microorganism that can promote plant growth, enhance soil health, and produce various enzymes, antibiotics, and vitamins. However, B. subtilis can also have some negative impacts on animal and plant health, depending on the strain, host, and environmental conditions.

Animal diseases

Bacillus subtilis has been isolated from different cases of bovine and ovine abortions, but it hasn’t been implicated as the causative agent of such infections. Besides, B. subtilis has been associated with cases of bovine mastitis, but the number of cases of mastitis caused by B. subtilis is low when compared to other species. B. subtilis has also been shown to be capable of infecting and resulting in the death of 2nd instar larvae of the mosquito. The ability of B. subtilis to cause infections in insects indicates the potential of the use of B. subtilis as a biocontrol agent.

A possible virulence factor of B. subtilis is toxin production as the bacteria produces the enzyme lecithinase which has been shown to be involved in food poisoning. Besides, B. subtilis also produces an extracellular toxin called subtilisin which is a proteinaceous compound capable of causing allergic reactions in some individuals. These reactions are often observed in immunocompromised individuals when they are exposed to such toxins regularly. The cases of allergies and hypersensitivity reactions, including dermatitis and respiratory distress, are often observed after the use of laundry products that are made with the said toxin.

The virulence of B. subtilis, as well as B. subtilis toxins, is relatively low and it has been suggested that the bacteria do not produce significant quantities of the enzymes or toxins.

Plant diseases

Bacillus subtilis is not considered a plant pathogen, but there have been some reports regarding the involvement of B. subtilis in the soft rot of garlic cloves. Based on a report, it was assumed that B. subtilis might be involved in the broad open cancer ulcrea in maple trees. The occurrence of B. subtilis in both animals and plants is quite limited and is not the primary causative agent.

However, B. subtilis can also have beneficial effects on plant health by acting as a plant growth-promoting rhizobacterium (PGPR) that can confer biotic and abiotic stress tolerance to plants by induced systemic resistance (ISR), biofilm formation and lipopeptide production. As a part of bioremediating technologies, Bacillus spp. can purify metal contaminated soil. It acts as a potent denitrifying agent in agroecosystems while improving the carbon sequestration process when applied in a regulated concentration. Even though it harbours several antibiotic resistance genes (ARGs), it can reduce the horizontal transfer of ARGs during manure composting by modifying the genetic makeup of existing microbiota.

External inoculation of B. subtilis has both positive and negative impacts on the endophytic and semi-synthetic microbial community. Soil texture, type, pH and bacterial concentration play a crucial role in the regulation of all these processes. Soil amendments and microbial consortia of Bacillus produced by microbial engineering could be used to lessen the negative effect on soil microbial diversity.

The complex plant-microbe interactions could be decoded using transcriptomics, proteomics, metabolomics and epigenomics strategies which would be beneficial for both crop productivity and the well-being of soil microbiota.

Industrial uses / Applications of Bacillus subtilis

Bacillus subtilis is a versatile microorganism that has been widely used for industrial production of various biological agents, such as small molecule compounds, bulk chemicals, industrial enzymes, precursors of drugs and health products. B. subtilis has several advantages as an industrial cell factory, such as:

• It is a Gram-positive bacterium with a single cell membrane, which facilitates protein secretion and simplifies downstream processing.
• It has a highly efficient protein secretion system and an adaptable metabolism, which enable it to produce and secrete different types of proteins and metabolites.
• It has a clear genetic background and a simple and diverse genetic manipulation system, which allow for easy engineering and optimization of the production strains.
• It has a fast growth rate and a short fermentation cycle, which reduce the production costs and time.
• It is generally recognized as safe (GRAS) by the Food and Drug Administration, which ensures the safety of its products for human and animal consumption.

Some of the industrial applications of B. subtilis and its products are summarized in the following table:

Bacillus subtilis is a ubiquitous bacterium that is generally regarded as safe (GRAS) by the Food and Drug Administration. It is widely used as a model organism for cell differentiation and biotechnology, as well as a probiotic and biocontrol agent in agriculture and food industries. However, some strains of B. subtilis have been associated with opportunistic infections in humans and animals, as well as food poisoning and spoilage. The virulence factors of B. subtilis are not well understood, but some possible mechanisms have been suggested based on experimental studies and genomic analysis.

Toxin production

One of the possible virulence factors of B. subtilis is the production of toxins that can cause damage to host cells or tissues, or elicit immune responses. B. subtilis produces several extracellular enzymes that have proteolytic, lipolytic, or hemolytic activities, such as lecithinase, subtilisin, phospholipase C, and sphingomyelinase. These enzymes can degrade host cell membranes, release cellular contents, or interfere with signal transduction pathways. Some of these enzymes have been implicated in food poisoning outbreaks caused by B. subtilis-contaminated foods.

B. subtilis also produces a lipopeptide antibiotic called surfactin, which has antimicrobial, antiviral, antitumoral, and hypocholesterolemic properties. Surfactin can disrupt the integrity of bacterial and eukaryotic membranes by forming pores or micelles. Surfactin can also modulate the immune system by inducing cytokine production, activating macrophages, and stimulating natural killer cells. However, surfactin can also cause adverse effects in some individuals, such as allergic reactions, inflammation, or cytotoxicity.

Adhesion and invasion

Another possible virulence factor of B. subtilis is the ability to adhere to and invade host cells or tissues, which can facilitate colonization, dissemination, and evasion of host defenses. B. subtilis has been shown to bind to various host receptors, such as fibronectin, collagen, laminin, vitronectin, and integrins. These receptors are involved in cell adhesion, migration, differentiation, and signaling. B. subtilis can also produce extracellular polysaccharides or capsules that can mediate adhesion or protect the bacteria from phagocytosis or complement-mediated killing.

B. subtilis can also invade host cells by different mechanisms, such as endocytosis, membrane ruffling, or actin polymerization. These mechanisms can allow B. subtilis to enter epithelial cells, endothelial cells, macrophages, or leukocytes. Once inside the host cells, B. subtilis can survive and replicate by escaping from the phagosomes or lysosomes, or by modulating the intracellular environment. B. subtilis can also induce apoptosis or necrosis of host cells by activating caspases or releasing toxins.

Spore formation

A third possible virulence factor of B. subtilis is the formation of endospores that can resist harsh environmental conditions and persist in the host for long periods of time. B. subtilis forms spores when exposed to nutrient limitation or stress signals. The spores are composed of a core containing DNA and ribosomes surrounded by a cortex layer and a proteinaceous coat. The spores are highly resistant to heat, desiccation, radiation, chemicals, antibiotics, and disinfectants. The spores can also germinate when they encounter favorable conditions or specific germinants.

The spore formation of B. subtilis can contribute to its virulence by allowing the bacteria to survive in unfavorable environments or during host immune responses. The spores can also be dispersed by air currents or water droplets and reach new hosts or niches. The spores can also trigger inflammatory responses in the host by activating pattern recognition receptors or complement system. The spores can also cause diseases such as anthrax or tetanus when they germinate and produce toxins in the host tissues.

Conclusion

Bacillus subtilis is a versatile bacterium that has both beneficial and harmful effects on humans and animals. The virulence factors of B. subtilis are not well characterized but may involve toxin production, adhesion and invasion of host cells or tissues, and spore formation. These factors may enable B. subtilis to cause opportunistic infections or food poisoning in some cases. However, most strains of B. subtilis are harmless or even beneficial for human health and industrial applications.

Bacillus subtilis is a Gram-positive, rod-shaped bacterium that is widely distributed in various environments, such as soil, water, air, and animal and human body surfaces. It is generally regarded as a safe and beneficial microorganism that can promote plant growth, enhance soil health, and produce various enzymes, antibiotics, and vitamins. However, B. subtilis can also have some negative impacts on animal and plant health, depending on the strain, host, and environmental conditions.

Animal diseases

Bacillus subtilis has been isolated from different cases of bovine and ovine abortions, but it hasn’t been implicated as the causative agent of such infections. Besides, B. subtilis has been associated with cases of bovine mastitis, but the number of cases of mastitis caused by B. subtilis is low when compared to other species. B. subtilis has also been shown to be capable of infecting and resulting in the death of 2nd instar larvae of the mosquito. The ability of B. subtilis to cause infections in insects indicates the potential of the use of B. subtilis as a biocontrol agent.

A possible virulence factor of B. subtilis is toxin production as the bacteria produces the enzyme lecithinase which has been shown to be involved in food poisoning. Besides, B. subtilis also produces an extracellular toxin called subtilisin which is a proteinaceous compound capable of causing allergic reactions in some individuals. These reactions are often observed in immunocompromised individuals when they are exposed to such toxins regularly. The cases of allergies and hypersensitivity reactions, including dermatitis and respiratory distress, are often observed after the use of laundry products that are made with the said toxin.

The virulence of B. subtilis, as well as B. subtilis toxins, is relatively low and it has been suggested that the bacteria do not produce significant quantities of the enzymes or toxins.

Plant diseases

Bacillus subtilis is not considered a plant pathogen, but there have been some reports regarding the involvement of B. subtilis in the soft rot of garlic cloves. Based on a report, it was assumed that B. subtilis might be involved in the broad open cancer ulcrea in maple trees. The occurrence of B. subtilis in both animals and plants is quite limited and is not the primary causative agent.

However, B. subtilis can also have beneficial effects on plant health by acting as a plant growth-promoting rhizobacterium (PGPR) that can confer biotic and abiotic stress tolerance to plants by induced systemic resistance (ISR), biofilm formation and lipopeptide production. As a part of bioremediating technologies, Bacillus spp. can purify metal contaminated soil. It acts as a potent denitrifying agent in agroecosystems while improving the carbon sequestration process when applied in a regulated concentration. Even though it harbours several antibiotic resistance genes (ARGs), it can reduce the horizontal transfer of ARGs during manure composting by modifying the genetic makeup of existing microbiota.

External inoculation of B. subtilis has both positive and negative impacts on the endophytic and semi-synthetic microbial community. Soil texture, type, pH and bacterial concentration play a crucial role in the regulation of all these processes. Soil amendments and microbial consortia of Bacillus produced by microbial engineering could be used to lessen the negative effect on soil microbial diversity.

The complex plant-microbe interactions could be decoded using transcriptomics, proteomics, metabolomics and epigenomics strategies which would be beneficial for both crop productivity and the well-being of soil microbiota.

Bacillus subtilis is a versatile microorganism that has been widely used for industrial production of various biological agents, such as small molecule compounds, bulk chemicals, industrial enzymes, precursors of drugs and health products. B. subtilis has several advantages as an industrial cell factory, such as:

• It is a Gram-positive bacterium with a single cell membrane, which facilitates protein secretion and simplifies downstream processing.
• It has a highly efficient protein secretion system and an adaptable metabolism, which enable it to produce and secrete different types of proteins and metabolites.
• It has a clear genetic background and a simple and diverse genetic manipulation system, which allow for easy engineering and optimization of the production strains.
• It has a fast growth rate and a short fermentation cycle, which reduce the production costs and time.
• It is generally recognized as safe (GRAS) by the Food and Drug Administration, which ensures the safety of its products for human and animal consumption.

Some of the industrial applications of B. subtilis and its products are summarized in the following table:

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