Priestia

Priestia is a genus of predominantly Gram-positive, aerobic, rod-shaped, endospore-forming bacteria in the family Sutcliffiellaceae.^[1][2] The genus was proposed in 2020 by reclassifying a distinct clade comprising seven Bacillus species based on comprehensive phylogenomic analyses and the identification of unique conserved signature indels (CSIs) in proteins such as oligoribonuclease NrnB, which distinguish these bacteria from other Bacillaceae members.^[3] Named in honor of British microbiologist Fergus G. Priest for his contributions to Bacillus systematics, Priestia has the type species Priestia megaterium (formerly Bacillus megaterium), a soil-dwelling bacterium known for its large cell size and spore-forming ability.^[4][3] As of 2025, the genus includes ten validly named species, such as P. aryabhattai, P. flexa, and P. koreensis, many of which are mesophilic, motile, and found in diverse environments including soil, rhizospheres, and air.^[4] Priestia species, particularly P. megaterium, are notable for their biotechnological potential, including the production of vitamin B12, biopolymers like polyhydroxybutyrate (PHB), and recombinant proteins, as well as applications in plant growth promotion, biocontrol against pathogens, and bioremediation of heavy metals.^[5][6]

Taxonomy

Classification

The genus Priestia belongs to the domain Bacteria, phylum Bacillota, class Bacilli, order Caryophanales, family Sutcliffiellaceae, as per recent taxonomic revisions based on phylogenomic analyses of type strains.^[2] Originally placed within the family Bacillaceae upon its establishment in 2020, Priestia was reassigned to the newly proposed Sutcliffiellaceae following a comprehensive reevaluation of the order Caryophanales using 1080 high-quality genome sequences, which highlighted distinct phylogenetic clusters among Bacillaceae genera.^[3][2] Priestia was proposed as a novel genus by Gupta et al. in 2020 to accommodate a monophyletic clade of species previously classified under Bacillus, distinguished through phylogenomic and comparative genomic analyses that identified unique molecular markers separating it from the core Bacillus lineage.^[3] These analyses included whole-genome comparisons and 16S rRNA gene sequence similarities, revealing average nucleotide identity values below 95% and digital DNA-DNA hybridization below 70% with Bacillus species, supporting the generic separation.^[3] The type species is Priestia megaterium (formerly Bacillus megaterium), selected for its representative position within the clade and historical significance as a well-studied soil bacterium.^[3] As of 2024, the genus Priestia comprises 10 validly published species, reflecting its establishment with the reclassification of several former Bacillus species such as P. aryabhattai, P. flexa, and P. koreensis based on shared genomic signatures.^[4] No additional species have been validly added since 2021, though ongoing genomic studies continue to refine boundaries within the Sutcliffiellaceae.^[2]

Etymology and History

The genus name Priestia is a New Latin feminine noun derived from the surname of Prof. Fergus G. Priest (1948–2019), a British microbiologist at Heriot-Watt University, Edinburgh, honoring his extensive contributions to the taxonomy and systematics of Bacillus species.^[4] Prior to 2020, species now assigned to Priestia were classified within the polyphyletic genus Bacillus, which encompassed diverse bacterial groups lacking clear phylogenetic boundaries. This classification stemmed from early 19th- and 20th-century descriptions based primarily on morphological traits, such as spore formation and Gram-positive staining, rather than genomic evidence. For instance, the type species Priestia megaterium was originally described as Bacillus megaterium by Anton de Bary in 1884 from soil samples, highlighting its large cell size (hence "megaterium").^[3] In 2020, Radhey S. Gupta and colleagues proposed Priestia as a novel genus in the family Bacillaceae, based on phylogenomic analyses of over 300 genomes and comparative studies identifying 17 distinct clades within Bacillus. These analyses, using methods like multi-locus sequence alignments and whole-genome phylogenies, demonstrated that the Priestia clade formed a robust, monophyletic group separate from core Bacillus lineages (e.g., the Subtilis and Cereus clades), supported by unique conserved signature indels in proteins. This reclassification addressed the polyphyly of Bacillus by elevating well-demarcated clades to genus level, with the proposal published in the International Journal of Systematic and Evolutionary Microbiology (DOI: 10.1099/ijsem.0.004475). As a result, 10 species were transferred to Priestia, including B. megaterium, B. aryabhattai, B. endophytica, and B. qingshengii, establishing P. megaterium as the type species.^[3][4]

Description

Morphology

Priestia species are rod-shaped bacilli, typically measuring 0.5–1.2 μm in width and 1.2–4.0 μm in length.^[7][8] Members of the genus are mostly Gram-positive, though exceptions exist, such as P. flexa, which is Gram-variable, while P. koreensis is Gram-positive.^[9][10] Priestia bacteria are endospore-producing, with spores that are central or subterminal and ellipsoidal in shape.^[11][8] The genus is mostly motile, with motility achieved via peritrichous flagella.^[8][12] On agar media, colonies of Priestia are circular, convex, and range from creamy white to beige in color, attaining diameters of 2–5 mm after 24–48 hours of incubation.^[13][11]

Physiology and Growth Characteristics

Priestia species are primarily aerobic, though some exhibit facultative anaerobic growth under certain conditions.^[14] These bacteria are chemoorganotrophic, deriving energy from the oxidation of organic compounds and utilizing a variety of carbohydrates such as glucose and starch, as well as amino acids, as carbon and nitrogen sources.^[15] Growth is supported in nutrient-rich media like nutrient broth or minimal salts media supplemented with glucose.^[16] The genus is mesophilic, with optimal growth temperatures ranging from 28 to 37°C, enabling robust proliferation under moderate environmental conditions typical of soil and plant-associated habitats.^[15] Priestia species can tolerate a broad temperature spectrum, growing from as low as 5°C to 48°C, and certain strains demonstrate psychrotrophic traits, allowing survival and slow growth in cooler environments.^[17] pH tolerance is similarly versatile, with optimal growth at neutral to slightly alkaline levels of 6.5–7.5 and viable growth spanning pH 5.0 to 9.0, reflecting adaptations to diverse terrestrial niches.^[15] Biochemical assays indicate that Priestia species are generally catalase-positive, facilitating the decomposition of hydrogen peroxide and contributing to oxidative stress resistance. Oxidase activity is variable across strains, with some testing negative, which aids in distinguishing species within the genus during identification.^[18] These physiological traits underscore the genus's resilience and potential for biotechnological exploitation in varied settings.

Phylogeny and Molecular Signatures

Phylogenetic Position

Priestia constitutes a monophyletic clade within the family Sutcliffiellaceae (previously classified in Bacillaceae), distinctly separated from Bacillus sensu stricto, as demonstrated by phylogenetic reconstructions using 16S rRNA gene sequences. This distinction is robustly supported by whole-genome phylogenomic analyses, including trees based on 650 core Bacillaceae proteins and 87 conserved proteins specific to the order Bacillales, where Priestia branches independently with high bootstrap support (100% SH-like values).^[3][2] Within the order Bacillales, Priestia occupies a position as a sister group to other Bacillaceae genera, such as the emended Bacillus (limited to the subtilis and cereus clades) and Peribacillus, reflecting deep evolutionary divergence within the family. Genomic data for Priestia species reveal consistent features that align with their phylogenetic placement, including chromosome sizes ranging from 4.0 to 5.7 Mb and G+C contents of 35–38 mol%, as observed in type strains like Priestia megaterium ATCC 14581 (5.7 Mb, 37.9 mol%). The original clade encompassed seven species; as of 2025, the genus includes ten species, maintaining the monophyletic nature supported by the original molecular markers.^[4] These phylogenetic inferences are additionally bolstered by conserved signature indels unique to the clade.

Conserved Signature Indels

The genus Priestia is defined by two conserved signature indels (CSIs) identified in the protein oligoribonuclease NrnB, a member of the DHH superfamily, which are exclusive to all sequenced members of this taxon.^[3] These include a 1 amino acid insertion at positions 87–124 and a 4 amino acid insertion at positions 203–251, flanked by conserved residues that confirm their specificity.^[3] These CSIs are absent in Bacillus species and other genera within the family Bacillaceae, providing diagnostic molecular markers that distinguish Priestia.^[3] These molecular signatures were discovered through comparative analyses of protein sequences from over 300 genomes representing Bacillus and related Bacillaceae species, enabling the robust demarcation of 17 distinct clades.^[3] As synapomorphies—shared derived characters—the CSIs corroborate the monophyly of the Priestia clade, which encompasses seven species including the type species P. megaterium, and support its elevation to genus status independent of the emended genus Bacillus.^[3] This approach complements phylogenomic trees based on concatenated core proteins, offering lineage-specific synapomorphies that enhance taxonomic reliability beyond sequence similarity.^[3] The presence of these CSIs in conserved regions of oligoribonuclease NrnB, an enzyme involved in RNA degradation, underscores their potential as tools for rapid identification of Priestia in genomic and metagenomic studies.^[3] While their exact functional contributions remain to be elucidated, such indels in essential proteins often correlate with adaptive traits unique to the lineage.^[3]

Species

Type Species

Priestia megaterium serves as the type species for the genus Priestia, which was established through a comprehensive phylogenomic reclassification of Bacillus species clades in 2020. Originally described as Bacillus megaterium by Anton de Bary in 1884, the species was named for its notably large cell size, distinguishing it from other bacilli at the time. This reclassification into the novel genus Priestia was based on robust genomic analyses identifying distinct monophyletic clades within Bacillaceae, with P. megaterium anchoring the genus due to its representative phylogenetic position and conserved molecular signatures.^[3] Morphologically, P. megaterium consists of large, rod-shaped cells measuring 1.2–1.5 μm in width and 3–5 μm in length, which are Gram-positive, aerobic, and capable of forming endospores. These spores contribute to the bacterium's resilience, allowing survival in harsh conditions. The species is ubiquitous across diverse environments, including soil, marine and freshwater sediments, air, and plant rhizospheres, reflecting its broad ecological adaptability.^[19] As a foundational species, P. megaterium holds significant value as a model organism in microbiological research, particularly for investigating sporulation mechanisms and enzyme production pathways. Its genome has been extensively sequenced, with the strain QM B1551 featuring a 5.1 Mb chromosome harboring approximately 5,300 genes, complemented by seven plasmids totaling 417 kb. This genomic framework has facilitated studies on gene regulation and protein expression, underscoring its role in advancing bacterial genetics.^[19][20]

Other Recognized Species

In addition to the type species Priestia megaterium, the genus Priestia encompasses nine other validly recognized species, all reclassified from the genus Bacillus in 2020 based on shared conserved signature indels and phylogenomic analyses that delineate a distinct clade within the Bacillaceae family. These species are primarily aerobic, endospore-forming rods, but they differ in Gram-stain reactions, motility, habitat preferences, and specialized tolerances.^[3]

• Priestia abyssalis, originally described as Bacillus abyssalis, is a moderately halophilic species isolated from seawater, characterized by its ability to grow in up to 10% NaCl and form ellipsoidal endospores; it exhibits optimal growth at 30–37 °C and pH 7–8.
• Priestia aryabhattai, formerly Bacillus aryabhattai, was isolated from air samples collected at high altitudes (up to 41 km), demonstrating remarkable resistance to arsenic (tolerating up to 10 mM As(V)) and UV radiation (surviving doses >500 J/m²), with motile cells bearing peritrichous flagella and optimal growth at 28–30 °C.
• Priestia endophytica, reclassified from Bacillus endophyticus, is an endophytic species recovered from plant roots, notable for its non-motile nature, production of indole-3-acetic acid, and tolerance to salt (up to 5% NaCl), thriving at 25–30 °C and pH 6–8.
• Priestia filamentosa, previously Bacillus filamentosa, features elongated, filamentous cells that can branch under certain conditions, isolated from forest soil; it is Gram-positive, motile, and grows optimally at 30 °C with a pH range of 6–9.
• Priestia flexa, derived from Bacillus flexus, displays flexible, rod-shaped morphology with Gram-variable staining and peritrichous flagella for motility; commonly found in marine and soil environments, it tolerates up to 7% NaCl and grows at 25–40 °C.^[12]
• Priestia iocasae, formerly Bacillus iocasae, is a halotolerant marine species isolated from coastal sediment, capable of growth in 0–12% NaCl, with ellipsoidal endospores and optimal conditions at 30 °C and pH 7–10; it is non-motile and oxidase-negative.
• Priestia koreensis, reclassified from Bacillus koreensis, uniquely stains Gram-negative despite its firmicute phylogeny, isolated from soil; it is motile, forms subterminal spores, and grows at 10–45 °C with a preference for neutral pH.
• Priestia paraflexa, originally Bacillus paraflexus, is a spore-forming rod from compost, exhibiting high salt tolerance (up to 7% NaCl) and broad temperature range (15–42 °C), with motile cells and growth at pH 5–11.
• Priestia qingshengii, formerly Bacillus qingshengii, was isolated from soil contaminated with crude oil, showing hydrocarbon degradation potential, Gram-positive staining, motility via peritrichous flagella, and optimal growth at 30–37 °C and pH 7.
• Priestia taiwanensis, formerly Bacillus taiwanensis, is a Gram-positive, aerobic, rod-shaped, endospore-forming bacterium isolated from soil in Taiwan; it is mesophilic with optimal growth at 30 °C and pH 7.0.^[21]

No additional species have been validly published and recognized in the genus Priestia since 2021, though ongoing genomic studies continue to refine boundaries within the Bacillaceae.^[4]

Ecology and Distribution

Habitats

Priestia species are primarily found in soil environments, where they constitute a significant portion of the aerobic, spore-forming bacterial communities in nutrient-rich terrestrial habitats. Most isolates, such as those of Priestia megaterium, have been recovered from various soil types, including agricultural and rhizospheric soils associated with plants like cotton, willow, and tomato.^[9][22][23] These bacteria thrive in aerobic conditions with ample organic matter, reflecting their role as common soil dwellers capable of utilizing diverse carbon sources.^[24] Beyond soil, Priestia species inhabit a range of other natural environments, including marine sediments and coastal zones. Strains like Priestia megaterium have been isolated from mangrove-inhabited sediments in the Indian Ocean along the Bagamoyo coast in Tanzania, as well as from coastal sediment samples in Veraval, India, indicating adaptation to saline and sediment-rich marine interfaces.^[25][26] Additionally, Priestia species have been isolated from extreme deep-sea hadal zones, such as the Java Trench sediments at depths exceeding 6,000 m, demonstrating adaptation to high-pressure, low-oxygen conditions.^[1] They occur in plant rhizospheres, where endophytic and associative forms colonize root zones, and in animal feces, such as those from livestock and insects, facilitating nutrient cycling in these organic matrices.^[1][27] Aerial habitats are also documented, notably with Priestia aryabhattai isolated from air samples at altitudes of 40–41.4 km, highlighting their dispersal via atmospheric currents.^[28] The genus exhibits a global distribution, with isolates reported from diverse locales including India, Korea, Japan, Tanzania, and Saudi Arabia, underscoring their cosmopolitan nature across continents.^[26][23][25] Priestia species are abundant in aerobic, nutrient-rich settings due to their spore-forming ability, which enables persistence and survival in fluctuating or harsh environmental conditions.^[24][1] Isolation from these habitats typically involves collecting environmental samples—such as soil, sediments, or air—and applying heat treatment to select for heat-resistant endospores, followed by cultivation on nutrient media.^[27][29]

Adaptations and Interactions

Priestia species exhibit notable adaptations that enable survival in challenging environmental conditions. As Gram-positive, aerobic, rod-shaped bacteria, they form endospores, which provide resistance to desiccation, heat, and other stresses typical of soil and aerial environments. Endospore formation is a conserved trait across the genus, allowing persistence in dry or high-temperature settings where vegetative cells would not survive.^[30] For instance, certain strains demonstrate tolerance to arsenic through mechanisms such as intracellular accumulation and volatilization, with P. aryabhattai showing resistance up to 100 mM arsenate.^[31] Additionally, P. aryabhattai isolated from stratospheric air, suggesting potential adaptations to high UV exposure.^[32] Some species, such as P. aryabhattai KX-3 from East Antarctic soils, display psychrotrophy, enabling growth at low temperatures such as 10°C, which supports colonization of cold environments.^[33] In terms of biotic interactions, Priestia species are commonly plant-associated, frequently colonizing the rhizosphere through chemotaxis toward root exudates like myo-inositol, facilitating non-pathogenic associations that benefit host plants without causing disease in humans or animals. They exhibit potential antagonism against plant pathogens, such as Erwinia amylovora, by producing antibiotics and other inhibitory compounds that suppress competitor growth. Priestia species employ survival strategies suited to their niches, relying on aerobic metabolism to thrive in oxygenated soil layers and forming biofilms that enhance adherence and protection in sedimentary environments. Biofilm production, mediated by genes like lapA, allows aggregation and resistance to environmental fluctuations in such settings. Ecologically, Priestia species function as decomposers in soil ecosystems, contributing to carbon cycling by enhancing microbial community-mediated sequestration and breakdown of organic matter, as observed with P. aryabhattai improving alfalfa rhizosphere carbon retention.

Biotechnological Applications

Industrial and Bioremediation Uses

Priestia megaterium serves as a versatile microbial chassis in industrial biotechnology due to its robust growth, GRAS status, and ability to produce high-value compounds under optimized fermentation conditions. This species has been engineered for the production of vitamin B12 (cobalamin), where metabolic pathway optimizations enable yields up to 0.2 µg/mL through low-oxygen cultivation strategies involving over 25 enzymatic steps. Additionally, P. megaterium accumulates polyhydroxybutyrate (PHB), a biodegradable bioplastic, with optimized processes using glycerol or industrial residues achieving polymer contents up to 49.6% of cell dry weight, highlighting its potential for sustainable plastics manufacturing.^[34][35][36] The large cell size and secretion capabilities of P. megaterium make it an effective host for recombinant protein expression, often surpassing Escherichia coli in yield for extracellular proteins via plasmid-based systems like the xylose-inducible promoters. For instance, it has been used to produce α-amylase enzymes at scalable levels through fed-batch fermentation, optimizing IPTG induction based on biomass ratios for applications in food processing and detergents. Similarly, high yields of proteases from P. megaterium strains support their use in laundry formulations and starch hydrolysis, with production enhanced by statistical bioprocess designs.^[37][38][39] In bioremediation, Priestia species demonstrate tolerance to environmental contaminants, facilitating cleanup of polluted sites. P. aryabhattai accumulates arsenic intracellularly, with evidence of ars operon involvement in resistance, reducing bioavailability in contaminated soils and promoting its use in phytoremediation consortia. Strains like P. megaterium exhibit resistance to heavy metals such as zinc and cadmium, biosorbing these ions from aqueous solutions through surface binding and precipitation mechanisms. Furthermore, P. megaterium MF3 degrades over 90% of the mycotoxin zearalenone in contaminated media within 24-72 hours, primarily via extracellular enzymes that cleave the lactone ring, offering a biological solution for fungal toxin removal in agriculture and feed.^[31][40][41] Whole-cell biocatalysis with Priestia leverages its reductase enzymes for stereoselective transformations in pharmaceutical synthesis. P. aryabhattai IICT-BC-1279 catalyzes the regio- and stereospecific 17β-reduction of androst-4-ene-3,17-dione to testosterone with high enantiomeric excess (>99% ee), utilizing NAD(P)H-dependent pathways suitable for scalable steroid intermediate production. The spore-forming stability of Priestia enhances process robustness in such applications by maintaining activity under varying conditions.^[42]

Plant Growth Promotion and Agricultural Roles

Priestia species, particularly P. megaterium and P. aryabhattai, function as plant growth-promoting rhizobacteria (PGPR) through multiple direct and indirect mechanisms that enhance crop productivity. These bacteria solubilize insoluble phosphates in the soil, converting them into plant-available forms via the secretion of organic acids such as gluconic acid, thereby improving phosphorus uptake in crops like alfalfa and mustard.^[43][44] Additionally, they produce siderophores that chelate iron and other micronutrients, facilitating their acquisition by plant roots under nutrient-limited conditions, as demonstrated by Priestia sp. LWS1 in selenium-contaminated soils.^[45] Indole-3-acetic acid (IAA) synthesis by strains like P. megaterium AIOASP1 and P. aryabhattai BPR-9 promotes root elongation and lateral branching, leading to increased nutrient absorption and overall plant vigor in various crops.^[43][46] In biocontrol applications, Priestia strains exhibit antifungal activity against soil-borne pathogens, suppressing diseases through antagonism and the induction of systemic resistance. For instance, P. megaterium JR48 inhibits Xanthomonas campestris pv. campestris, the causative agent of black rot in crucifers, by reinforcing salicylic acid (SA)-mediated defense pathways in plants, reducing lesion area by 46% and bacterial populations by 77% in greenhouse trials.^[47] Similarly, P. megaterium KW16 demonstrates strong inhibition of Rhizoctonia solani in oilseed rape via diffusible antifungal compounds and enzymes, enhancing plant immunity without phytotoxicity.^[6] These mechanisms also extend to pathogens like Fusarium species, where siderophore-mediated iron competition limits fungal growth.^[48] Field and greenhouse applications of Priestia inoculants have shown consistent benefits for crop health and yield. Inoculation of apple roots with P. megaterium B1L5 enriches the rhizosphere microbiome, improving nutrient uptake and alleviating apple replant disease symptoms in orchard soils.^[49] For cruciferous crops, P. megaterium JR48 seed priming boosts phosphorus availability by 1.86-fold and enhances drought tolerance through better root development.^[43][47] In contaminated farmlands, P. aryabhattai serves as a dual-purpose inoculant, promoting alfalfa growth under agrochemical stress while aiding bioremediation by increasing soil carbon sequestration and microbial diversity.^[50][51] Strains like P. megaterium mj1212 further improve drought resilience in cowpea and mustard by modulating root transcriptome for enhanced water and nutrient efficiency.^[44][52]