Priestia megaterium: Features, Uses, and Significance

Priestia megaterium is one of the largest known bacteria and among the most versatile workhorses in microbiology, with applications spanning biodegradable plastics, pharmaceutical enzymes, agricultural biostimulants, and environmental cleanup. Originally classified as Bacillus megaterium for well over a century, it was reclassified into the new genus Priestia in 2020 based on genomic evidence, so much of the older literature still uses the former name. What makes this organism remarkable is less any single trait and more the sheer range of useful things it can do, a breadth that researchers are still mapping.

A Bacterium You Can Almost See

Most bacteria are microscopic specks even under a decent lab microscope, but P. megaterium stands out. Its cells can reach roughly 1.5 by 4 micrometers, large enough to be clearly visible under low magnification and roughly a hundred times the volume of a typical Escherichia coli cell. The species name itself means “big beast.” It is a Gram-positive, rod-shaped, spore-forming bacterium found in extraordinarily diverse habitats: agricultural soils, river sediments, plant roots, seawater, even honey and dried foods. That ecological generalism is a clue to its metabolic flexibility.

The genome of P. megaterium is moderately large for a bacterium, typically around 5.7 megabases with a GC content near 38%.¹ Many strains also carry multiple plasmids, some of which encode stress-response genes, sugar transporters, and oxidative-stress enzymes like catalase.² This accessory genome likely explains why the species thrives in such varied and sometimes hostile environments.

Making Biodegradable Plastic

One of the headline applications of P. megaterium is its natural ability to produce polyhydroxyalkanoates (PHAs), a family of polyesters that bacteria accumulate as internal carbon and energy reserves. PHAs behave like conventional plastics in many respects but are fully biodegradable, making them attractive for packaging, medical devices, and single-use items. The specific type P. megaterium tends to produce is polyhydroxybutyrate (PHB), a well-characterized PHA with good thermoplastic properties.

What makes this bacterium especially interesting for PHA production is that it can grow on cheap, waste-derived carbon sources rather than expensive refined sugars. Different strains show a wide range of production characteristics when cultured on glycerol, a common biodiesel waste product: cell dry weights from about 0.5 to 5.7 grams per liter, PHA content from 4% to 42% of cell mass, and dramatic variation in the molecular weight of the resulting polymer.³ That molecular weight variation matters for downstream material properties, because higher molecular weight generally means stronger, more flexible films.

Some strains perform considerably better. When researchers optimized growth conditions for a strain called ASL11 using glycerol, they tripled PHB content to about 50% of cell dry weight compared to what the same strain achieved on glucose.⁴ And a recently characterized strain called GM-4 pushed the boundaries further still, reaching a PHA content of 63% on glucose, and when fed sugarcane molasses and corn steep liquor (both industrial waste streams), it yielded about 9.8 grams of PHA per liter. That is among the highest PHA outputs reported for any wild bacterial strain using waste feedstocks at laboratory flask scale.⁵

The appeal here is economic. PHA production costs have long been a barrier to replacing petroleum-based plastics, and feeding bacteria cheap agricultural or industrial waste instead of purified glucose could significantly close that gap. P. megaterium’s ability to grow well on glycerol, molasses, and similar low-value carbon sources gives it a genuine commercial edge.

A Protein Production Platform

Beyond making its own useful polymers, P. megaterium serves as a host organism for producing proteins that researchers design and insert into it. This is the world of recombinant protein production, and it is central to modern biotechnology: everything from industrial enzymes to pharmaceutical proteins is made by engineering a host microbe to churn out the desired molecule.

P. megaterium has several advantages in this arena. It naturally secretes large quantities of protein into its surrounding medium, which simplifies purification because you do not have to break open the cells and fish your target molecule out of the debris. It grows readily on inexpensive carbon sources. And crucially, a mature commercial toolkit exists for it: nearly 30 expression vectors (the DNA vehicles that carry the gene of interest) are commercially available, supporting both intracellular and secreted production at gram-per-liter scale.⁶ That toolkit has been built up over decades, and it gives P. megaterium a level of engineering convenience that few other Gram-positive bacteria can match.

One commercially significant enzyme that P. megaterium produces is penicillin G acylase (PGA). This enzyme is a workhorse in the pharmaceutical industry, used to manufacture semisynthetic beta-lactam antibiotics (a category that includes many widely prescribed drugs). Structural studies of P. megaterium PGA have captured the enzyme in several distinct states during its catalytic cycle, providing atomic-level detail of how it cleaves and reassembles the chemical bonds needed for antibiotic synthesis.⁷ Understanding these details helps engineers design improved enzyme variants for industrial use.

Vitamin B12 by Fermentation

P. megaterium is one of the relatively few organisms that naturally synthesizes vitamin B12 (cobalamin), a molecule with a notoriously complex structure that is expensive to make by chemical synthesis. The bacterium produces B12 through the anaerobic branch of the tetrapyrrole biosynthetic pathway, which is the same general route it uses to make the heme groups needed for its cytochrome P450 enzymes.

Wild-type P. megaterium produces only trace amounts of B12, but both process optimization and genetic engineering have pushed yields dramatically higher. A two-step cultivation strategy that optimized fermentation conditions achieved a roughly 760-fold increase in B12 output compared to the unoptimized baseline.⁸ On the genetic engineering side, overexpressing a key enzyme in the biosynthetic pathway (a modified version of the enzyme that catalyzes the first committed step) increased intracellular B12 concentrations by roughly 33-fold over the wild type.⁹ These improvements are stepping stones toward industrial-scale microbial B12 production, though current commercial B12 fermentation still relies primarily on other organisms.

Boosting Plant Growth in Agriculture

If P. megaterium has a single most-studied ecological role, it is probably as a plant growth-promoting rhizobacterium (PGPR). This term describes soil bacteria that colonize the root zone and help plants grow through various mechanisms. P. megaterium excels at several of these simultaneously.

The most consistent trait across strains is phosphate solubilization. Much of the phosphorus in soil is locked up in insoluble mineral forms that plant roots cannot access. P. megaterium secretes organic acids, particularly gluconic acid and oxalic acid, that dissolve these mineral phosphates and release plant-available phosphorus.¹⁰ A strain isolated from rhizosphere soil demonstrated phosphate-solubilizing ability in the range of roughly 24 to 86 milligrams per liter and was shown to carry the specific genes responsible for producing gluconic acid.¹¹ This phosphate-freeing capability is particularly valuable in acidic soils, like those under tea plantations, where aluminum and iron bind phosphorus tightly.

But phosphate solubilization is only part of the story. Genomic and physiological studies of various strains have confirmed a wider suite of plant-beneficial traits:

Phytohormone production: Many strains synthesize indole-3-acetic acid (IAA), a plant growth hormone that stimulates root development and elongation.
Siderophore secretion: These iron-chelating molecules help scavenge iron from the soil and make it available to plant roots.
Zinc solubilization: Some strains dissolve zinc-containing minerals, freeing another essential micronutrient for plant uptake.
Biofilm formation: Colonizing roots as a biofilm helps the bacterium persist in the rhizosphere and deliver its benefits over time.

A strain called B1, isolated from healthy apple roots, was shown through both genomic analysis and lab experiments to possess genes for chemotaxis (swimming toward plant root signals), flagella construction, and all of the traits listed above.¹² This strain lives inside root tissue as an endophyte, giving it intimate access to the plant.

Some strains go beyond growth promotion into active disease suppression. Strain KW16, studied in oilseed rape, showed biocontrol activity against Rhizoctonia solani, a damaging fungal pathogen. Its genome revealed the genetic basis for producing lytic enzymes that degrade fungal cell walls, along with volatile compounds and siderophores that inhibit pathogen growth.¹³ That combination of fertilizer-like and fungicide-like effects in a single organism is what makes P. megaterium attractive as a biological alternative to synthetic agrochemicals.

Cleaning Up Pollutants and Heavy Metals

The same metabolic versatility that lets P. megaterium thrive in diverse soils also equips it for environmental remediation. Strains isolated from mine tailings have been shown to tolerate moderate concentrations of heavy metals while continuing to produce siderophores, IAA, and other plant-growth-promoting compounds.¹⁴ These strains carry genes for metal efflux pumps that actively expel toxic metal ions from the cell, a survival strategy that keeps the bacterium functional in contaminated soils. The practical vision is to pair metal-tolerant P. megaterium strains with metal-accumulating plants for phytoremediation, where the bacterium helps the plant establish itself in harsh soil while the plant’s roots pull metals out of the ground.

P. megaterium also shows up in the degradation of organic pollutants. A consortium of four bacterial species including P. megaterium achieved 87% removal of glyphosate, one of the world’s most widely used herbicides, along with efficient turnover of its main breakdown product.¹⁵ In another line of research, a strain designated P-7 was isolated from cotton fields in Xinjiang, China, that had been covered with plastic mulch for years. This strain could break down di-n-butyl phthalate (DBP), a plasticizer that leaches from agricultural films into soil. Genomic and metabolic analysis revealed that the bacterium uses esterases to crack the phthalate ester bonds, then funnels the fragments through several degradation pathways until they enter the cell’s central metabolism.¹⁶

Another area involves mineral weathering. A comparative study of P. megaterium strain PMA2-2 showed that its ability to dissolve biotite (a mineral found in many soils) was linked to coordinated activity of genes on both its chromosome and its plasmids, producing organic acids, exopolysaccharides, and biofilm components that collectively attack the mineral surface.¹⁷ Mineral dissolution may sound esoteric, but it is how nutrients like potassium and iron get released from rock into soil, so bacteria that speed the process contribute meaningfully to soil fertility.

Genetic Engineering Tools

For all of P. megaterium’s natural talents, unlocking its full potential requires the ability to edit its genome precisely. Historically, genetic manipulation of P. megaterium lagged behind model organisms like E. coli or Bacillus subtilis. That has changed. Researchers adapted the CRISPR-Cas9 system for use in P. megaterium by modifying promoters and ribosomal binding sites and adding a counterselection system that ensures the editing plasmid is properly removed after the job is done. The resulting tool achieved 100% editing efficiency when tested by disrupting an endogenous gene.¹⁸

This is a significant enabling technology. With reliable genome editing, researchers can knock out competing metabolic pathways to funnel more carbon toward PHA production, insert heterologous genes for new biosynthetic capabilities, or fine-tune regulatory circuits to optimize protein secretion. The combination of an efficient CRISPR system and the existing library of expression vectors puts P. megaterium in a strong position as a chassis organism for synthetic biology, particularly for applications where a Gram-positive host is preferred over E. coli.

Safety and Pathogenicity

A question that hovers over any microorganism used in agriculture, food-adjacent industries, or pharmaceutical manufacturing is whether it can cause disease. P. megaterium has a long track record of being considered non-pathogenic or at most of very low virulence.¹⁹ It lacks the toxin genes that make its relative Bacillus cereus dangerous and does not carry the anthrax-related virulence factors of Bacillus anthracis.

That said, “non-pathogenic” is not an absolute guarantee. Rare case reports exist of P. megaterium causing soft tissue infections, nearly always in immunocompromised patients or in the context of traumatic wounds. These cases are uncommon enough to be individually published as clinical curiosities. For healthy individuals and standard industrial or agricultural use, the species is widely considered safe, and many strains are used in commercially sold biofertilizer products without regulatory concern.

How It Reshapes the Soil Around Plant Roots

One of the more nuanced aspects of P. megaterium’s agricultural role involves not just what it does directly for the plant, but how it changes the broader microbial community in the root zone. A study using apple plantlets found that inoculating soil with P. megaterium strain B1L5 significantly shifted the composition of the bacterial community around the roots, enriching it with other known beneficial genera including Pseudomonas, Sphingomonas, Lysobacter, and Flavobacterium. The inoculant itself colonized the roots within days but was no longer detectable about a month later, yet the community changes it triggered persisted.²⁰

In apple replant disease soil, which is notorious for suppressing new apple tree growth, inoculated plants showed fewer blackened root tips compared to untreated controls. Fungal communities, interestingly, were largely unaffected by the inoculation, suggesting that P. megaterium’s influence operates primarily through bacterial community restructuring rather than direct antifungal activity in this system. The fact that a transient visitor can leave a lasting imprint on the rhizosphere microbiome hints at ecological mechanisms that go beyond the simple “bacterium feeds plant” story, and it raises the possibility that the timing and formulation of bioinoculant application may matter as much as the strain selected.

Where the Research Is Heading

The volume of published work on P. megaterium has accelerated sharply in recent years. Genome sequencing projects are cataloguing strain diversity from environments as varied as desert mine tailings, tropical tea plantations, and temperate orchards. Each new genome reveals different plasmid complements, different suites of stress-resistance genes, and different secondary metabolite clusters, underscoring how much functional diversity exists within a single species name. Comparative genomic studies are beginning to connect these genetic differences to measurable phenotypic outcomes, like why one strain dissolves minerals more effectively or produces more PHA than another.

On the applied side, the convergence of cheap genome sequencing, efficient CRISPR editing, and growing demand for bio-based materials and sustainable agriculture creates obvious opportunities. Engineered P. megaterium strains optimized for PHA production on agricultural waste could contribute to displacing petroleum-based plastics. Strains tailored for specific crops or soil types could reduce reliance on synthetic fertilizers and pesticides. And the organism’s long history of safe use, combined with its unusually large and easy-to-manipulate cells, keeps it accessible to labs that might not have the resources to work with more finicky chassis organisms. P. megaterium is not flashy in the way that CRISPR babies or mRNA vaccines capture public attention, but quietly, across dozens of applied fields, this big beast of a bacterium keeps proving useful.