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The Frankia bacteria are not involved nor do they grow in water-damaged buildings. This group of bacteria is presented because they are classified as Actinobacteria (Actinomycetes) and are in the family Frankiacea. They are classified as Frankia alni and Frankia sp.

Description and Significance

Frankia is a nitrogen-fixing bacterium that lives in the soil and has a symbiotic relationship with many plants. The actinomycete Frankia is of fundamental and ecological interests for several reasons including its wide distribution, its ability to fix nitrogen, differentiate into sporangium and vesicles (specialized cell for nitrogen-fixation) and to nodulate plants from about 24 genera. 

Species of the Frankia Genus are Gram positive bacteria. 

Frankia sp. are filamentous nitrogen-fixing bacterium that grow by branching and tip extension and thus resemble the antibiotic-producing Streptomyces sp. They live in the soil and have a symbiotic relationship with certain woody angiosperms, called actinorhizal plants. 

During growth, the Frankia sp. produce three cell types: sporangiospores, hyphae, and diazo-vesicles (spherical, thick walled, lipid-enveloped cellular structures). The diazo-vesicles are responsible for the supplying of sufficient nitrogen to the host plant during symbiosis. 

Frankia supplies most or all of the host plant nitrogen needs without added nitrogen and thus can establish a nitrogen-fixing symbiosis with host plants where nitrogen is the limiting factor in the growth of the host. Therefore, actinorhizal plants colonize and often prosper in soils that are low in combined nitrogen. Symbiosis of this kind adds a large proportion of new nitrogen to several ecosystems such as temperate forests, dry chaparral, sand dunes and mine wastes.

The organisms in symbiotic relationships with plant hosts are necessary for the health and survival of certain plants. They assist in creating and transporting certain root hormones, controlling pathogens and nematodes, root exploration, water retention, mineral uptake and resource sharing. 

Frankia specifically fixes nitrogen in the air and produces molecules that other plants can use. 
Frankia on ceanothus roots

Frankia bacteria on Ceanothus roots

Frankia alni

Frankia alni

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