Actinomycetota

Actinomycetota. Formerly known as Actinobacteria, this major phylum of Gram-positive bacteria is renowned for its high guanine-cytosine content in DNA and filamentous growth resembling fungi. Its members are ubiquitous in soil, where they play a critical role in decomposition and biogeochemical cycles, and include genera of immense medical and industrial significance, such as the antibiotic-producing Streptomyces. The phylum encompasses a vast diversity, from pathogens like Mycobacterium tuberculosis to commensals in the human gut.

Taxonomy and classification

The phylum Actinomycetota was formally proposed by J.P. Euzéby following advances in 16S ribosomal RNA gene sequencing, which clarified its phylogenetic distinctiveness. It is placed within the domain Bacteria and is divided into several classes, including the medically and industrially pivotal Actinomycetia, which contains the order Actinomycetales. Other major classes are Acidimicrobiia, Coriobacteriia, and Thermoleophilia. The taxonomy is continually refined through techniques like whole-genome sequencing and multilocus sequence analysis, with the List of Prokaryotic names with Standing in Nomenclature serving as a key authoritative resource.

Morphology and cell structure

Morphology within the phylum is highly diverse, ranging from coccoid cells in Micrococcus to complex, branching filaments in Streptomyces that form a substrate mycelium and aerial hyphae. These filaments exhibit true branching, akin to fungi, and differentiate into chains of exospores. The cell envelope is characterized by a thick peptidoglycan layer, and many genera possess unique lipids like mycolic acid in the Mycobacterium genus, contributing to acid-fast staining properties. Some members also produce an external S-layer or capsule.

Metabolism and ecology

Actinomycetota are primarily heterotrophs, renowned as dominant decomposers of complex organic polymers like chitin, cellulose, and keratin in terrestrial ecosystems. They contribute significantly to the carbon cycle and nitrogen fixation, with some genera like Frankia forming symbiotic root nodules with plants such as alder. They are predominantly aerobes, though some, like Actinomyces, are facultative anaerobes. Their metabolic versatility allows them to thrive in diverse habitats, from Antarctic soils to hot springs and the human microbiome.

Pathogenicity and medical importance

Several genera are notable human and animal pathogens. Mycobacterium tuberculosis, the causative agent of tuberculosis, and Mycobacterium leprae, causing leprosy, are of profound global health importance. Corynebacterium diphtheriae produces the toxin responsible for diphtheria. Actinomyces israelii can cause actinomycosis, while Nocardia asteroides is associated with nocardiosis. Conversely, the gut microbiota member Bifidobacterium is considered probiotic, and the phylum is the source of the majority of clinically used antibiotics, including streptomycin and tetracycline.

Industrial and biotechnological applications

The phylum, particularly the genus Streptomyces, is the workhorse of the industrial production of bioactive compounds. Beyond antibiotics, these include antifungal agents, antiparasitic drugs like ivermectin, and immunosuppressants such as rapamycin. Actinomycetota are also exploited for the production of enzymes like proteases and cellulases used in detergents and biofuel production. Their ability to produce geosmin gives soil its characteristic earthy smell, and they are used in the fermentation of foods and in bioremediation of environmental pollutants.

Genomics and evolution

Genomes within Actinomycetota are typically large, often exceeding 8 megabase pairs, and are rich in biosynthetic gene clusters encoding pathways for secondary metabolites. The genome of Streptomyces coelicolor was one of the first sequenced for the phylum, revealing a linear chromosome. Evolutionary studies using molecular clock analyses suggest an ancient origin, with adaptations leading to saprotrophic lifestyles and complex developmental cycles. Horizontal gene transfer events, particularly involving antibiotic resistance genes, have played a significant role in their evolution and adaptation.