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Indian Journal of Biotechnology Vol. 18, October 2019, pp 293-304 House of industrially important bioactive metabolites: A review on actinobacteria Naushin Bano1, Saba Siddiqui2, Mohammad Amir1 and Roohi1* 1Department of Bioengineering, Integral University, Lucknow, India 2Integral Institute of Agricultural Science and Technology, Integral University, Lucknow, India Received 14 January 2019; revised 17 July 2019; accepted 30 July 2019 The actinomycetes are Gram-positive bacteria present in natural habitats, most profoundly in soil, freshwater, sea water and composts. Actinomycetes have high GC (guanine-cytosine) content and characterized by a complex life cycle. The morphology of actinomycetes looks like fungi and the reproduction takes place either by hyphae fragmentation or by spore production in the mycelium. Actinomycetes serves as a large pool for secondary metabolites production with potent biological activities and therapeutic properties like antifungal, antibiotics, enzymatic, plant growth promoting rhizobacteria (PGPR), in xenobiotics and bioremediation. These bioactive metabolites production can be enhanced using high-throughput fermentation and by combinatorial biosynthesis during optimization. Present research should be focused on the potential of less explored actinomycetes from extreme environments that can be utilized as sustainable herald for green biotechnology. In this review, habitat, life cycle and production optimization of actinomycetes in selective media are explained but article is emphasized on biotechnological potential of actinomycetes that are relevant for industries, agriculture and allied sectors. Keywords: Actinomycetes, secondary metabolites, enzymes, antibiotics, bioremediation, applications. Introduction proteobacteria) that live in and around the roots of the Actinomycetes are also called actinobacteria and it plant4. Actinomycetes are one of the most important is made up of two words i.e. aktis which means microorganisms in microbial diversity and they are lightning and mykes means fungus that was initially explored from a range of habitats and unusual classified as an interposed group between bacteria and environment that produces wide variety of useful fungi1. Actinomycetes are Gram-positive bacteria that secondary metabolites like antibiotics and immune have been placed within the phylum of actinobacteria suppressive compounds that are commercially while the class is actinobacteria, subclass is important5. The bioactive potential of these bacteria actinobacteridae and order is actinomycetales which facilitates their continued existence even in distress currently consists of 10 suborders and it has more and favorable ecological conditions. The general than 30 families and approximately 160 genera2. The characteristics of actinobacteria are summarized in important source of isolating endophytic Table 1. actinomycetes is medicinal plants, which can induce secondary metabolites production which possess Structure of Actinomycetes Actinomycetes are filamentous and spore forming immense applications at industrial level. The bacteria that are habitually found in soils. These endophytic microorganisms do not cause any direct bacteria contain ~57-75% of GC content in their and harmful effect while colonizing living internal 14 3 DNA . Actinomycetes are recognized by their tissue . Although endophytic and rhizospheric are branching and filamentous growth pattern which both plant-associated bacteria that give valuable forms a mycelium, a very large colony of filaments effects on the host plants but endophytic bacteria are which split individually to form rods or spheroidal more reliable and specific as they are present within shapes, called bacillus. The lysine is present in the the tissues of the plant. Pyrosequencing method is peptidoglycan of actinomyces species. The other used to reveal a wide range of bacteria (like internal structures are thallus which is a tissue like actinobacteria, bacteroides, verrucomicrobia and mass that grows in cultures and mycelium which is —————— tangled mass of hyphae that is found in nature and *Author for correspondence Tel: +91-9450358872 some actinomycetes can form spores. They are mainly E-mail: [email protected] chemo-organotrophic that grows at neutral pH but are 294 INDIAN J BIOTECHNOL, OCTOBER 2019 Table 1 — Characteristics of actinomycetes Genus Aerobic/ GC content Size Morphology Motility Other References Anaerobic (%) (µm) characteristics Actinoplanes Aerobic 72-73 They have Branched mycelium Their They are found in soil 6 irregular size and aerial growth is spores are and decaying plant and shape, little and no motile and material, they are highly forms fragments flagella are colored and often sporangia polar contain hyphae in which are 5-18 palisade arrangement µm in diameter Arthrobacter Aerobic 59-70 08-12 × 10-80 Young cells are Usually Their growth cycle is 7 arranged in irregular non-motile rod-coccus, catalase rod shapes, and older positive, found in soil cells are small cocci and contains respiratory metabolism Bifidobacterium Anaerobic 55-67 05-13 × 15-80 Usually curved and Non-motile Their cells are in clubbed 8 varied shaped rods or branched shaped, pairs are often in V- arrangement They ferment carbohydrates to accelerate lactate production but no CO2 generated, catalase negative Corynebacterium Facultative 51-63 03-08 × 15-80 The rods are straight Non-motile Cells are often arranged 9 anaerobic or slightly curved in V formation or in with clubbed ends palisades of parallel cells, they are catalase positive, fermentative and contains polyphosphate granules Frankia Aerobic to 66-71 05-20 They have no aerial Non-motile They fix nitrogen, 6 micro mycelium and sporangio- usually contains type- aerophillic vegetative hyphae, spores III cell walls, mostly and form multicellular symbiotic strains with sporangia angiosperms Micrococcus Aerobic 64-75 05-20 Cocci are in pairs Non-motile Usually red or yellow 10 and clusters are colour colonies catalase uneven positive, respiratory metabolism present, found in soils and primarily on mammalian skin Mycobacterium Aerobic 62-70 02-06 × 10-10 The rods are straight Non- motile Catalase positive, they 11 and slightly curved, form readily fragmented some are branched, filaments, high lipid acid fast and non content walls, some sporing species are parasitic and found in soil and water Nocardia Aerobic 64-72 05-12 Extensive Non- motile They are catalase 12 vegetative hyphae positive and forms aerial that can fragmented hyphae It contains type- into rod shaped and IV cell walls and found cocci forms in soil Streptomyces Aerobic 69-78 05-20 Mycelium is Non-motile They form discrete 13 vegetative and lichenoid or leathery extensively colonies, that often are branched and the pigmented, respiratory chains of three to metabolism is present and many spores are as a nutrient they use formed by aerial many organic compounds mycelium present in soil ROOHI et al: ACTINOBACTERIA AND BIOACTIVE METABOLITES 295 also capable to grow in alkalophilic and acidophilic grow in all directions from a central point and conditions and some of them are phototrophic, produce an appearance that has been compared with autotrophic and heterotrophic. When actinomycetes is the rays of sun or of a star. Therefore, the grown on an agar-surface, then it forms a network of actinomycetes are also called ‘ray fungi’17. They often hyphae like branch form, growing both inside and produce complicated designs and resemble some of outside of the agar. The on-the-surface hyphae are the drawings in modern art exhibitions. The called aerial hyphae and the under-surface hyphae are protoplasm of the young hyphae appears to be called substrate hyphae. Septa normally divide the undifferentiated, but the older parts of the mycelium hyphae into long cells about 20 mm and longer, and show definite granules, vacuoles and nuclei. Many having bacterial chromosomes (nucleoids). There are actinomycetes at first produce a very delicate, widely the aerial hyphae that expand over the substratum and branched, mycelium that may embed itself into the reproduce asexually. Mainly actinomycetes are non- soil, or if grown in culture, into the solid medium. motile but restricted to flagellated spores, when This kind of mycelium is therefore called the motility is present. The composition of cell wall ‘substratum or primary mycelium’ as shown in the varies significantly among various groups and is of Figure 1. considerable taxonomic significance15. Actinomycetes After a period of growth, hyphae of a different kind lacks membrane bound organelles and nucleus develop which raise themselves up from the because they are like bacteria containing double substratum mycelium and grow in the air. These are stranded DNA (ds-DNA) and ribosomes in cytosol. called as aerial hyphae and the corresponding Many bacterial cells also contain plasmids, which are mycelium is the aerial or secondary mycelium. The circular DNA molecules. A particular ribosomal color of the aerial mycelium may be white, yellow, RNA sequence i.e. 16S rRNA is present in violet, red, blue, green, or grey and many form actinomycetes. There is an incomplete mycelia pigments that are excreted into the medium, and development is present during active growth in slightly wider than the substratum mycelium. The actinobacteria, while in, well-matured actinobacteria, aerial hyphae possess an extra cell wall layer called as there are two types of mycelium i.e., rhizoids sheath. The hyphal tip undergoes septa formation in

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