Paenibacillus

Paenibacillus is a genus of facultative anaerobic, endospore-forming bacteria, originally included within the genus Bacillus and then reclassified as a separate genus in 1993.^[2] Bacteria belonging to this genus have been detected in a variety of environments such as: soil, water, rhizosphere, vegetable matter, forage and insect larvae, as well as clinical samples.^[3][4][5][6] The name reflects this fact: Latin paene means almost, and so the Paenibacilli are literally almost Bacilli. The genus includes P. larvae, which is known to cause American foulbrood in honeybees, the P. polymyxa, which is capable of fixing nitrogen and therefore is used in agriculture and horticulture, the Paenibacillus sp. JDR-2 which is known to be a rich source of chemical agents for biotechnology applications and pattern forming strains such as P. vortex and P. dendritiformis discovered in the early 90s,^{[7][8][9][10][11]} which are known to develop complex colonies with intricate architectures^{[12][13][14][15][16]} as is illustrated in the pictures.

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  Picture 1: A colony generated by P. vortex sp. bacteria. The colony diameter is 5cm and the colors indicate the bacteria density (bright yellow for high density). The bright dots are the vortices described in the text.

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  Picture 2: A colony generated by the Branching (Tip splitting) morphotype bacteria of P. dendritiformis. The colony diameter is 6cm and the colors indicate the bacteria density (darker shade for higher density).

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  Picture 3: A colony generated by the Chiral morphotype bacteria of P. dendritiformis. The colony diameter is 5cm and the colors indicate the bacteria density (bright yellow for high density). Note that the branches are curly with well defined handedness.

Importance

There has been a rapidly growing interest in Paenibacillus spp. since many were shown to be important^[17][18][19] for agriculture and horticulture (e.g. P. polymyxa), industrial (e.g. P. amylolyticus), and medical applications (e.g. P. peoriate). These bacteria produce various extracellular enzymes such as polysaccharide-degrading enzymes and proteases, which can catalyze a wide variety of synthetic reactions in fields ranging from cosmetics to biofuel production. Various Paenibacillus spp. also produce antimicrobial substances that affect a wide spectrum of micro-organisms^[20][21][22] such as fungi, soil bacteria, plant pathogenic bacteria and even important anaerobic pathogens as Clostridium botulinum.

More specifically, several Paenibacillus species serve as efficient plant growth promoting rhizobacteria (PGPR). PGPR competitively colonize plant roots and can simultaneously act as biofertilizers and as antagonists (biopesticides) of recognized root pathogens, such as bacteria, fungi and nematodes.^[23] They enhance plant growth by several direct and indirect mechanisms. Direct mechanisms include phosphate solubilization, nitrogen fixation, degradation of environmental pollutants and hormone production. Indirect mechanisms include controlling phytopathogens by competing for resources such as iron, amino acids and sugars, as well as by producing antibiotics or lytic enzymes.^[24][25] Competition for iron also serves as a strong selective force determining the microbial population in the rhizosphere. Several studies show that PGPR exert their plant growth-promoting activity by depriving native microflora of iron. Although iron is abundant in nature, the extremely low solubility of Fe³⁺ at pH 7 means that most organisms face the problem of obtaining enough iron from their environment. To fulfill their requirements for iron, bacteria have developed several strategies, including (i) the reduction of ferric to ferrous ions, (ii) the secretion of high-affinity iron-chelating compounds, called siderophores, and (iii) the uptake of heterologous siderophores. P. vortex's genome for example,^[26] harbors many genes which are employed in these strategies, in particular it has the potential to produce siderophores under iron limiting conditions.

Despite the increasing interest in Paenibacillus spp. genomic information of these bacteria is lacking. More extensive genome sequencing could provide fundamental insights into pathways involved in complex social behavior of bacteria, and can discover a rich source of genes with biotechnological potential.

Candidatus Paenibacillus glabratella is causing white nodules and high mortalities of Biomphalaria glabrata freshwater snails.^[27] This is potentially important because Biomphalaria glabrata is an intermediate host transmitting schistosomiasis.^[27]

Pattern formation, self-organization and social behaviors

Several Paenibacillus species can form complex patterns on semi-solid surfaces. Development of such complex colonies require self-organization and cooperative behavior of individual cells while employing sophisticated chemical communication.^{[7][8][12][14][15][28][29][30]} Pattern formation and self-organization in microbial systems is an intriguing phenomenon and reflects social behaviors of bacteria^[29][31] that might provide insights into the evolutionary development of the collective action of cells in higher organisms.^{[7][29][32][33][34][35][36]}

Pattern forming in P. vortex

One of the most fascinating pattern forming Paenibacillus species is P. vortex, self-lubricating, flagella driven bacteria.^[26] P. vortex organizes its colonies by generating modules, each consisting of many bacteria, which are used as building blocks for the colony as a whole. The modules are groups of bacteria that move around a common center at about 10 µm/s.

Pattern forming in P. dendritiformis

An additional intriguing pattern forming Paenibacillus species is P. dendritiformis, which is known to be able to generate two different morphotypes^{[7][8][12][13][14][15]} – the Branching (or tip-splitting) morphotype and the Chiral morphotype that is marked by curly branches with well defined handedness (see pictures).

These two pattern forming Paenibacillus strains exhibit many distinct physiological and genetic traits including β-galactosidase-like activity causing colonies to turn blue on X-gal plates and multiple drug resistance (MDR) (including septrin, penicillin, kanamycin, chloramphenicol, ampicillin, tetracycline, spectinomycin, streptomycin and mitomycin C. Colonies that are grown on surfaces in Petri dishes exhibit several folds higher drug resistance in comparison to growth in liquid media. This particular resistance is believed to be due to a surfactant-like liquid front that actually forms a particular pattern on the Petri plate.

References

External links

• Paenibacillus Taxonomy
• Genome sequence of the pattern forming Paenibacillus vortex bacterium reveals potential for thriving in complex environments - manuscript
• Prof. Eshel Ben-Jacob home page
• Specific PCR for Paenibacillus genus based on rpoB gene
• Use of rpoB gene analysis for identification of nitrogen-fixing Paenibacillus species as an alternative to the 16S rRNA gene