Priestia megaterium is a fascinating and impactful bacterium, drawing attention from various scientific and industrial fields. This Gram-positive rod is notable for its exceptional size among bacteria and its remarkable ability to form spores. Originally classified as Bacillus megaterium, it underwent reclassification to its current genus, Priestia, based on modern genetic analysis. This reclassification reflects its distinct evolutionary lineage.
Distinctive Biological Features
Priestia megaterium exhibits several distinguishing biological characteristics. Its cell wall possesses a thick layer of peptidoglycan, which retains a purple stain during the Gram staining procedure. The bacterium is exceptionally large, with individual cells measuring up to 100 micrometers in length and 0.1 micrometers in diameter. To put its size into perspective, its volume can be over 60 cubic micrometers, making it more than 100 times larger than a common bacterium like Escherichia coli, which typically has a volume of approximately 0.5 cubic micrometers. These cells often appear in pairs and chains, linked by polysaccharides in their cell walls.
A significant survival mechanism for Priestia megaterium is its ability to form endospores. These are dormant, robust structures that allow the bacterium to endure harsh environmental conditions such as extreme temperatures, desiccation, and radiation. The spores are typically ellipsoidal to spherical and can be found centrally, paracentrally, or subterminally within the cell; notably, the sporangia, or mother cells, do not swell during spore formation. This bacterium is widely distributed in diverse natural environments, including soil, marine sediments, water, the upper atmosphere, and even the internal tissues of plants like cotton.
The reclassification from Bacillus to Priestia in 2020 by Gupta and colleagues was driven by comparative phylogenetic analyses and genomic sequencing, which revealed that P. megaterium belonged to a genetically distinct clade separate from other Bacillus species. The genus name Priestia honors microbiologist Fergus G. Priest for his extensive contributions to the systematics of Bacillus species.
Industrial and Biotechnological Uses
Priestia megaterium is a versatile cellular factory, producing and secreting proteins and other valuable compounds. It naturally produces a range of enzymes with significant commercial applications. It produces amylases, utilized in the food processing industry for starch hydrolysis, and proteases, employed in detergents and various food applications. It also contributes to the pharmaceutical sector by producing specific enzymes like cytochrome P450, useful in drug metabolism studies and characterized for its role in vitamin D3 hydroxylation.
Beyond enzymes, Priestia megaterium is a natural producer of cobalamin, commonly known as Vitamin B12, and has been used for its industrial production. This vitamin is essential for higher eukaryotes and is exclusively synthesized by certain bacteria and archaea in nature. The bacterium’s potential also extends to bioremediation, offering solutions for environmental clean-up. Some strains, including those formerly classified as Bacillus aryabhattai but now recognized as P. megaterium, demonstrate resistance to arsenic and UV radiation, suggesting applications in removing metal pollutants. The bacterium’s metabolic versatility allows it to break down various environmental pollutants, including hydrocarbons and certain pesticides.
Role in Agriculture
Priestia megaterium plays a substantial role in agriculture as a plant growth-promoting rhizobacterium (PGPR), benefiting plant health and soil fertility. It colonizes the root systems of plants, where it exerts several positive effects on plant growth and development. One primary mechanism is improving nutrient availability for plants. This bacterium is particularly effective at phosphate solubilization, converting insoluble inorganic phosphate into forms that plants can readily absorb. It achieves this by producing organic acids, such as gluconic acid, lactic acid, oxalic acid, acetic acid, and succinic acid, along with enzymes like phosphatases and phytases, which break down phosphorus-containing compounds in the soil.
Many strains of Priestia megaterium also contribute to plant nutrition by fixing atmospheric nitrogen, converting it into ammonia, a form usable by plants, thereby reducing the need for synthetic nitrogen fertilizers. The bacterium further promotes plant growth by synthesizing and secreting various plant hormones, including auxins and gibberellins, which stimulate root and shoot development. As a biocontrol agent, P. megaterium helps protect plants from diseases by outcompeting harmful pathogens for nutrients and niche space, and by producing antimicrobial compounds. For example, studies show its ability to induce plant resistance to crucifer black rot (Xanthomonas campestris pv. campestris) by reinforcing salicylic acid accumulation and response within the plant. The bacterium can also enhance plant tolerance to various abiotic stresses, such as drought, salinity, and nutrient deficiencies, making crops more resilient.
Significance in Scientific Research
Priestia megaterium serves as a compelling model organism in microbiology and molecular biology research. Its unusually large size makes it an advantageous subject for studying basic cellular processes. Researchers utilize P. megaterium to investigate complex phenomena such as cell wall synthesis, cell division, protein secretion pathways, and the intricate process of sporulation. Its considerable volume allows for easier observation and manipulation in laboratory settings.
The bacterium’s genetic tractability further enhances its value as a research tool. It is relatively easy to genetically modify, serving as an effective cloning host capable of accommodating plasmid vectors. A notable advantage is its natural lack of alkaline proteases, which could otherwise degrade recombinant proteins. This characteristic makes P. megaterium a safe and efficient platform for producing proteins for research and therapeutic purposes. Its generally non-pathogenic nature, often recognized with GRAS status, contributes to its safety and utility in molecular biology and genetic engineering research.