Bacillus siamensis

Bacillus siamensis is one of the most promising species in the panorama of microbiology applied to agriculture.

First described in 2010 from a sample isolated from salted crab (poo-khem) in Thailand, this Gram-positive, aerobic or facultatively anaerobic bacterium is part of the species complex Bacillus subtilis, specifically the operational group B. amyloliquefaciens.

From the point of view taxonomic, B. siamensis exemplifies the phylogenetic complexity of the genus Bacillus, which has been restructured by advances in comparative genomics.

Belonging to the domain Bacteria, phylum Bacillota, class Bacilli, order Bacillales, family Bacillaceae and genus Bacillus, its classification follows the International Code of Nomenclature of Prokaryotes (ICNP) and is registered in the NCBI Taxonomy with ID 659243.

The name "siamensis" honors the ancient name for Thailand (Siam), reflecting its geographic origin. Amended by Dunlap in 2015, the species has synonyms such as the heterotypic "Bacillus vanilla" (not validated), and is distinguished from close congeners, such as B. amyloliquefaciens e B. velezensis, due to low DNA-DNA hybridization (<70%), despite high similarities in 16S rRNA sequences (99,2–99,5%).

Analyses based on genes such as rpoB and core genomes reinforce its insertion in the amyloliquefaciens group, a clade adapted to rhizospheric niches, where genetic divergence supports unique phenotypic adaptations, such as salt tolerance.

A biology de Bacillus siamensis reveals a resilient and multifunctional bacterium. Its morphological, physiological, and genetic characteristics equip it to colonize hostile environments.

Morphologically, it exhibits rod-shaped cells (0,5–1,0 μm wide by 2,0–4,0 μm long), motile by peritrichous flagella and forming central or subterminal oval endospores, conferring resistance to thermal and chemical stresses. Its colonies on nutrient agar are circular, smooth and cream-colored, growing optimally at 30–37°C, pH 7,0 and 0–2% NaCl, with a wide tolerance (4–55°C, pH 4,5–9,0, up to 14% NaCl).

Biochemically, is catalase-positive, hydrolyzes starch and casein, reduces nitrate and produces acetoin, but not indoles or ferments lactose; chemotaxonomically, it stands out for menaquinone-7 (MK-7) and fatty acids such as anteiso-C15:0 (40–50%).

Genetically, its 3,9–4,1 Mb circular chromosome (G+C 41,4 mol%) encodes biosynthesis of antifungal lipopeptides (bacillomycin, fengycins, surfactins), phosphate solubilization, and nitrogen fixation, as well as clusters for abiotic resistance. Strains such as B28 illustrate this versatility, with genes that modulate plant responses to pathogens, positioning B. siamensis as an efficient plant growth-promoting microorganism (PGPR).

Ecologically, Bacillus siamensis is ubiquitous. It occupies niches ranging from agricultural soils to marine ecosystems, where it acts as a modulator of microbial communities.

Predominant in the rhizospheres of crops such as rice, soybeans, tomatoes, and bananas, it endophytically colonizes plant tissues, promoting symbioses that increase nutrient uptake and reduce reactive oxygen species (ROS). Its halotolerance makes it ideal for saline soils and mangroves, while isolation from termite nests and fermented foods (e.g., organic soybeans) suggests roles in biogeochemical cycles, such as fermentation and dissemination via insect vectors.

In interactions with plants, it competes for nutrients, elicits systemic defenses, and produces siderophores and auxins, reprogramming genes for greater resilience. This ecological dynamic is not isolated; it integrates B. siamensis into rhizosphere microbiomes, where it balances pathogenic and abiotic populations, fostering microbial biodiversity and environmental sustainability.

In the scope etiological, Bacillus siamensis subverts traditional expectations of pathogenicity associated with the genus Bacillus, acting predominantly as a beneficial antagonist.

Absent virulence factors such as toxins or adhesins, it does not cause infections in plants, humans or animals; on the contrary, its role is protective, inhibiting phytopathogens such as Fusarium oxysporum, magnaporthe oryzae e Colletotrichum spp.

Strains such as GP-P8 and QN2MO-1 reduce disease incidence by up to 70%, via mechanisms such as spatial competition, production of antimicrobial metabolites and induction of plant immunity.

In agricultural contexts, this translates into a reduction in damping-off (60% reduction) and anthracnose, promoting yields without chemical residues. Its indirect etiology reinforces the concept of integrated agriculture, where B. siamensis mitigates epidemiological risks in crop rotation.