# Priestia megaterium

Priestia megaterium (Bacillus megaterium prior to 2020) is a rod-like, Gram-positive, mainly aerobic, spore forming bacterium found in widely diverse habitats. It has a cell length up to 100 μm and a diameter of 0.1 μm, which is quite large for bacteria. The cells often occur in pairs and chains, where the cells are joined by polysaccharides on the cell walls.
In the 1980s, prior to the use of Bacillus subtilis for this purpose, P. megaterium was the main model organism among Gram-positive bacteria for intensive studies on biochemistry, sporulation, and bacteriophages. Recently, its popularity has started increasing in the field of biotechnology for its recombinant protein-production capacity.

## Characteristics

P. megaterium grows at temperatures from 3 to 45 °C, with the optimum around 30 °C. Some isolates from an Antarctic geothermal lake were found to grow at temperatures up to 63 °C. It has been recognized as an endophyte and is a potential agent for the biocontrol of plant diseases. Nitrogen fixation has been demonstrated in some strains of P. megaterium.
P. megaterium has been an important industrial organism for decades. It produces penicillin amidase used to make synthetic penicillin and several enzymes, such as amylases used in the baking industry and glucose dehydrogenase used in glucose blood tests. It also produces enzymes for modifying corticosteroids and several amino acid dehydrogenases. Further, it is used for the production of pyruvate, vitamin B$_{12}$ and molecules with fungicidal and antiviral properties. Several of these bioactive compounds are cyclic lipopeptides, belonging to the surfactin, iturin, and fengycin lipopeptide families, which are also produced by several other Bacillus species.
P. megaterium is known to produce poly-γ-glutamic acid. The accumulation of the polymer is greatly increased in a saline (2–10% NaCl) environment, in which the polymer comprises largely of L-glutamate (L-isomer content up to 95%). At least one strain of P. megaterium can be considered a halophile, as growth on up to 15% NaCl has been observed.
Phylogenetically, based on 16S rRNA, P. megaterium is strongly linked with B. flexus, the latter distinguished from P. megaterium a century ago, but only recently confirmed as a different species. P. megaterium has a complex plasmid content as well as some phenotypic and phylogenetic similarities with pathogens B. anthracis and B. cereus, although itself being relatively harmless.

## Isolation

P. megaterium is ubiquitous in the environment. In addition to being a common soil bacterium and an endophyte, it can be found in various foods (including honey and bee pollen, in which most microorganisms do not grow) and on a variety of surfaces, including clinical specimens, leather, paper, stone etc. It has also been isolated from cattle feces, emperor moth caterpillars, and greater wax moth frass.

## History of the name

The species was described by de Bary in 1884, who called it Bacillus megaterium, but did not give an etymology. However, some subsequent authors called it B. megatherium assuming the name was incorrectly spelled. This trend continues as many scientists still use the name B. megatherium, sowing confusion.
The name P. megaterium is a nominative noun in apposition (see Rule 12 of IBCN) and is formed from the Greek adjective mega, (μέγας , μεγάλη, μέγα) meaning "great", and a second word of unclear etymology. Three hypotheses of the epithet "megaterium" are possible:
* unintentional orthographic error (unlikely given the fact that de Bary and his students, consistently used the epithet "megaterium"), whereas it should have been megatherium, from therion (θηρίον, meaning "beast"), to mean "great beast".
* a contraction of "megabacterium" as speculated by Rippel in given the fact that de Bary called the bacterium with the nickname Grosstier or Grossvich
* stems from teras, teratos (τέρας, τέρατος, a neuter noun meaning omen or wonder or, indirectly, monster,) which could be interpreted to mean "great monster" (with the Neolatin name being formed incorrectly given that there is no evidence of a Greek third declension noun when converted into Latin becoming a second Latin declension using the nominative stem, which is "ter-" while the other case use the stem "terat-". If were converted into a third declension noun it would have been "megateras, -atis").
Consequently, it was decided in the first juridical opinion of the Bacteriological code that the name should remain "megaterium" given the unclear meaning.
The etymology listed in LPSN is, despite being not quite correct, a fusion of the first and third interpretation Gr. adj. megas, large; Gr. n. teras -atis, monster, beast; N.L. n. megaterium, big beast.
The species name megaterium has been applied to other genera.