Introduction

The Actinobacteria are a phylum of Gram-positive bacteria. They can be terrestrial or aquatic. They are of great economic importance to humans because agriculture and forests depend on their contributions to soil systems. In soil they help to decompose the organic matter of dead organisms so the molecules can be taken up anew by plants. While this role is also played by fungi, Actinobacteria are much smaller and likely doing not occupy the same ecological niche.

In this role the colonies often grow extensive mycelia, like a fungus would, and the name of an important order of the phylum, Actinomycetales (the actinomycetes), reflects that they were long believed to be fungi. Some soil actinobacteria (such as Frankia) live symbiotically with the plants whose roots pervade the soil, fixing nitrogen for the plants in exchange for access to some of the plant's saccharides. Other species, such as many members of the genus Mycobacterium, are important pathogens.

Characteristics of actinobacteria

Actinobacteria comprises a group of branching unicellular microorganisms, most of which are aerobic-forming mycelium known as substrate and aerial. They reproduce by binary fission or by producing spores or conidia, and sporulation of Actinobacteria is through fragmentation and segmentation or conidia formation. The morphological appearance of Actinobacteria is compact, often leathery, giving a conical appearance with a dry surface on culture media and is frequently covered with aerial mycelium.

Morphology

Morphology has been an important characteristic to identify Actinobacteria isolates, which was used in the first descriptions of Streptomyces species. This is made using various standard culture media, including International Streptomyces Project (ISP). For nonstreptomycetes or rare Actinobacteria, strains maintained on ATCC Medium No.172 (NZ-amine glucose starch agar) were used. Various morphological observations, including germination of spores, elongation and branching of vegetative mycelium, formation of aerial mycelium, color of aerial and substrate mycelium, and pigment production, have been used to identify Actinobacteria. Light microscopy was used to study the formation of aerial mycelium and substrate mycelium, and scanning electron microscopy was used to study the spores, the spore surface, and spore structure.

Mildiomycin, an antifungal metabolite isolated from cultures of Streptoverticillium rimofaciens Niida, is strongly active against several powdery mildews on various crops and inhibits fungal protein biosynthesis. The primary sites of action of these antibiotics are at locations where chitin synthesis occurs in the cell wall, there is cation leakage from mitochondria, inositol biosynthesis is occurring, or sites of protein and DNA synthesis.

The compounds mentioned above are a few examples of agroactive compounds isolated from Actinobacteria. Validamycin A was commercialized by Takeda for the control of pathogens in rice and other plants and as a tool for damping off diseases in vegetable seedlings. On the other hand, some secreted metabolites are cytotoxic and can include chemical structures such as macrolides, α-pyrones, lactones, indoles, terpenes, and quinones. For instance, resistomycin, a quinone-related antibiotic, has a unique structure and exhibits bactericidal and vasoconstrictive activity based on the inhibition of RNA and protein synthesis.

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