Actinobacteria

General

Actinobacteria are non-motile, gram-positive bacteria characterised by high guanine-plus-cytosine (G+C) DNA content ^[1]. The majority of Actinobacteria are free-living and are found ubiquitously in marine, fresh-water aquatic and terrestrial environments. Actinobacteria are particularly abundant in soil, where they perform many vital roles in the decomposition of organic matter, plant growth, and enrichment of the soil ecosystem ^[1]. Soil Actinobacteria are mesophilic, growing optimally at temperatures of 25-30°C, and prefer a neutral pH. Many saprophytic Actinobacteria also produce bioactive secondary metabolites which are the source of two-thirds of naturally derived antibiotics and numerous anticancer, antianthelmintic and antifungal compounds ^[1]. Bacteria belonging to this phylum are therefore of considerable biotechnological, medical and agricultural importance.

Actinobacteria is one of the largest taxonomic units within the Bacteria domain and is diversely comprised of human and plant pathogens, human and plant commensals, and free-living saprophytes. Well-known pathogenic Actinobacteria include those belonging to the genera Mycobacterium, Corneybacterium, Norcardia and Tropheryma, which cause disease in humans ^[1]. The majority of Actinobacteria are soil-dwelling and are widely distributed across varied ecological environments: from mangrove swamps to the Antarctic Tundra ^[2]. Soil Actinobacteria include the genera Micromonopspora and Streptomyces, both of which are prolific producers of antimicrobial compounds. Soil microbial populations are composed largely of the genus Streptomyces, which accounts for 95% the Actinomycetes strains isolated from soil ^[1]. Streptomyces also produce the vast majority of antibiotic compounds sourced from Actinobacteria ^[1][3].

Members of the Actinobacteria phyla display a diverse range of morphologies, including coccoid, rod-coccoid and fragmenting hyphal forms ^[1]. Many Actinobacteria are filamentous and mycelium-forming via tip-extension and hyphal branching ^[1]. Such mycelial Actinobacteria are referred to as Actinomycetes (“ray” and “fungus”) as they superficially resemble fungi, with many reproducing through sporulation ^[1].

Morphology and Lifecycle

Actinobacteria display varied morphology and lifecycles depending on the environmental niche in which they reside.

The Actinomycetes have a complex lifecycle involving significant morphological transitions which begins with a germinating spore. Germ-tubes emerge from the spore by tip-elongation and expand to form a branching multichromosomal filamentous network known as a vegetative mycelium. This is the first phase of growth, with the vegetative mycelium often expanding deep into the surrounding environment. The second phase of growth is initiated in response to nutrient depletion amongst other signals such as quorum-sensing and environmental stresses ^[4]. During this stage secondary metabolites are produced, and unbranched aerial hyphae grow upwards and outwards of the aqueous vegetative mycelium ^[1]. Aerial hyphae are sporogenic, forming septal cross walls which separate them into pre-spore compartments which will ultimately become spore chains. Spores are typically unigenomic and have thick spore-walls to protect the enclosed genetic material ^[4]. Mature spores are disseminated into the environment to begin the lifecycle anew. Actinomycetes Morphology and Lifecycle.

References

[1] Barka, E. A., Vatsa, P., Sanchez, L., Gaveau-Vaillant, N., Jacquard, C., Klenk, H.-P., … van Wezel, G. P. (2016). Taxonomy, Physiology, and Natural Products of Actinobacteria. Microbiology and Molecular Biology Reviews. https://doi.org/10.1128/mmbr.00019-15

[2] Qin, S., Li, W. J., Klenk, H. P., Hozzein, W. N., & Ahmed, I. (2019). Editorial: Actinobacteria in special and extreme habitats: Diversity, function roles and environmental adaptations, second edition. Frontiers in Microbiology. https://doi.org/10.3389/fmicb.2019.00944

[3] Manteca, Á., & Yagüe, P. (2019). Streptomyces as a Source of Antimicrobials: Novel Approaches to Activate Cryptic Secondary Metabolite Pathways. In Antimicrobials, Antibiotic Resistance, Antibiofilm Strategies and Activity Methods. https://doi.org/10.5772/intechopen.81812

[4] Flärdh, K. and Buttner, M. J. (2009) ‘Streptomyces morphogenetics: dissecting differentiation in a filamentous bacterium’, Nature Reviews Microbiology, 7(1), pp. 36–49. doi: 10.1038/nrmicro1968.