Classification of actinomycetes.
Biotechnological tools engaged in the bioremediation process are in reality, sophisticated and dynamic in character. For specialized reasons, a broad variety of such devices are employed to produce a safe and balanced environment free of all types of toxins and so make life simpler for humans on planet Earth. Actinomycetes is one of these extremely important and functionally helpful groups. They can be used for a variety of bioremediation objectives, including biotransformation, biodegradation, and many more. Actinomycetes are one of the most varied groups of filamentous bacteria, capable of prospering in a variety of ecological settings because to their bioactive capabilities. They’re famous for their metabolic diversity, which includes the synthesis of commercially useful primary and secondary metabolites. They produce a range of enzymes capable of totally destroying all of the constituents. They are well-known for their ability to produce bioactive secondary metabolites. Members of various genera of Actinomycetes show promise for application in the bioconversion of underutilized urban and agricultural waste into high-value chemical compounds. The most potential source is a wide range of important enzymes, some of which are synthesized on an industrial scale, but there are many more that have yet to be discovered. Bioremediation methods, which use naturally existing microbes to clear residues and contaminated regions of dangerous organic chemicals, are improving all the time. In the realm of biotechnological science, the potential of actinomycetes for bioremediation and the synthesis of secondary metabolites has opened up intriguing prospects for a sustainable environment.
- Sustainable environment
- Bioactive compounds
The word ‘actinomycete’ comes from the Greek words ‘atkis’ (ray) and’mykes’ (fungus), both of which contain bacterial and fungal features. Actinomycetes are gram-positive bacteria that generate spores and are found in nature aerobically. They are one of the most important taxonomic units among the 18 major lineages currently recognized within the domain bacteria. The Actinobacteria class is an important part of the microbial population in soils. Their metabolic variety and unique growth features make them ideal bioremediation agents [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]. Actinobacteria are a worldwide collection of microorganisms that may be found in a variety of natural habitats . They are Gram-positive bacteria with a high guanine plus cytosine (G1C) content base in DNA (5575 mol%), however new species have been discovered that do not follow this norm. This group is extremely diverse, as it contains a wide range of microorganisms that differ chemically, morphologically, and physiologically . This morphological variety is demonstrated by a constant shift from basal and bacilliform cells to hyphae that break and branch, generating aerial mycelium with lengthy chains of spores. Actiospores are generated as a result of nutrient deficiency and may withstand protracted desiccation . This capacity to sporulate is critical for their survival in the wild. Temperatures of 2530C and neutral pH are ideal for growth in most cases, however several species have been separated from harsh settings. Most of these bacteria are aerobic, although some may also be microaerophilic or anaerobic. They are heterotrophic, which means they can use both simple and complicated carbon sources . The creation of a significant number of biotechnologically significant metabolites (antibiotics, enzymes, enzyme inhibitors, immunomodulators, and so on) demonstrates physiological variety . Additionally, due to the formation of a metabolite called geosmin, they require a specific odor of damp soil as part of their unique traits.
1.1 Occurrence and habitat
Actinomycetes are the foremost abundant life style saprophytes that form thread-like filaments within the soil. They grow as hyphae like fungi liable for the characteristically “earthy” smell of freshly turned healthy soil. The actinomycetes exist in various habits in nature and represent a ubiquitous group of microbes cosmopolitan in natural ecosystems around the world. Actinomycetes are widely distributed in soil and ocean. There are many reports for isolation of actinomycetes from terrestrial soils [17, 18], marine ecosystem [19, 20], mangrove ecosystem [21, 22], composts, vermicomposts  Environmental factors influence the type and population of actinomycetes in soil. They are found both on mesophilic (25-300C) and thermophilic (400C) environments. The pH is also a major environmental factor determining the distribution and activity of actinomycete. Most of the actinomycetes grow at optimum pH around 7. Vasavada et al.  showed that pH, salinity, use of media and carbon and nitrogen sources affect the growth and antibiotic production by actinomycetes. Many mesophilic actinomycetes are active in compost in initial stages of decomposition. However the capacity for self-heating during decomposition provides ideal conditions for thermophilic actinomycetes. Actinomycetes diversity also can be influenced by the range of plant species grown thereon particular soil. Since different plants produce different chemical metabolites, so as to survive the microbes (actinomycetes during this case) got to adapt to the environment . As soil is the best source of Actinomycetes, much research has been focused on the soil ecology. They may be found in a variety of soils, both cultivated and uncultivated, fertile and infertile, in diverse parts of the world. pH is a major environmental element that determines the distribution and activity of soil Actinomycetes. Neutrophiles are found in less numbers in acidic soils with pH < 5.0, but acidophilic streptomycetes are plentiful. Many active mesophilic Actinomycetes may be found in compost. The research have recently been conducted in aquatic settings such as fresh and marine waters. Many researchers saw Actinomycetes as part of the native microflora of marine ecosystems, whereas others saw them as wash-in elements that persist as spores in marine and littoral sediments. Salt tolerance has been demonstrated in creatures from maritime settings. Actinomycetes have been studied for their presence, survival, and activity in a variety of severe settings. Streptomycetes, both acidophilic and aciduric, are common in acidic soils. Actinomycetes have been found in hot springs, marine sediments, and crater lakes, among other harsh habitats (soda lakes). Lonar Lake, a crater produced by meteorite impact, provides a unique opportunity. Because of the high sodium carbonate concentration, the lake water is salty and alkaline (pH 9.5 to 10.0).
The development of usually branched threads or rods distinguishes actinomycetes. In most cases, the hyphae are non-septate. The sporulating mycelium may be branching or non-branching, straight or spiral. Spherical, cylindrical, or oval spores are found. They resemble fungus morphologically, which might be related to the fact that their cell wall composition is similar to that of gramme positive bacteria. They have been separated from ordinary bacteria because of their filamentous form and cultural traits.
1.3 Actinobacteria: growth and reproduction
This diversified group has a lot of morphological differentiation, including septate and nonseptate multicellular strands and a filamentous-type structure. Strains often form compact colonies on solid culture medium, consisting of mycelium, a mass of hyphae pertaining to the microbe, and distinguishing into aerial and substrate mycelium. Actinobacteria have a modest growth rate in general. After 24 hours of incubation, a branching mycelium forms on the surface of a solid medium, which may be examined under a microscope; colonies form after 34 days, but mature aerial mycelium with actinospores occurs after 714 days as shown in Figure 1. Some strains that develop slowly may take up to a month to incubate. The development and stability of the substrate and aerial mycelium can be influenced by the culture medium composition. Colonies of Actinobacteria can be elevated or laid flat. Their texture ranges from incredibly soft to exceedingly hard and pasty. White, yellow, orange, pink, red, purple, blue, green, brown, and black are among the colors available. They might have smooth, grooved, wrinkled, granular, or flaky surfaces. Their appearance is frequently totally compact, or a mixture of both, with diverse developing zones in concentric rings of radial orientationThe colony size ranges from one millimeter to a few centimeters in diameter, depending on the species, age, and cultivation circumstances. Actinobacteria may grow on liquid medium as well, but only under certain circumstances. To achieve uniform growth, liquid cultures require agitation and aeration, as well as suspension in culture media. Actinobacteria are a common microbial community found in soil, with an average of 5631010 CFU/g of soil. Actinobacteria are found in the soil as latent spores that produce their mycelia only when specific environmental circumstances are ideal, such as nutrition availability, humidity, temperature, or physiological interactions with other microbes. These organisms are investigated for biotechnological applications, particularly in bioremediation of harmful chemicals, because to their metabolic variety and relationship with the environment. The interaction of Actinobacteria with accumulated pollutants in the environment, such as oil, rubber, plastics, pesticides, and heavy metals, has been investigated for more than 20 years [6, 10, 24]. The life cycle of the actinomycetes is shown Figure 1. Actinomycetes are mostly mycelioid and Gram-positive. They begin as unicellular creatures, but eventually evolve into branching filaments or hyphae, which multiply rapidly by generating new branches, forming the mycelium. As seen in Figure 1, this type of mycelium is known as “substratum or primary mycelium.” After a period of development, hyphae of a different sort emerge from the mycelium substratum and begin to grow in the air. Aerial hyphae and aerial or secondary mycelium are the terms used to describe these hyphae. Sheath is an additional cell wall layer seen on aerial hyphae.
2. Classification of actinomycetes
The “Bergey’s Manual of Systematic Bacteriology - 2nd edition” for Actinobacteria classification has five volumes, which contain internationally recognized names and descriptions of bacteria. Classification of Actinobacteria has been rearranged. In Volume 5, the phylum Actinobacteria is split into six classes, namely Actinobacteria, Acidimicrobiia, Coriobacteriia, Nitriliruptoria, Rubrobacteria, and Thermoleophilia. The class Actinobacteria is further divided into 16 orders that are Actinomycetales, Actinopolysporales, Bifidobacteriales, Catenulisporales, Corynebacteriales, Frankiales, Glycomycetales, Jiangellales, Kineosporiales, Micrococcales, Micromonosporales, Propionibacteriales, Pseudonocardiales, Streptomycetales, Streptosporangiales, and Incertae sedis. In the order of abundance in soils, the common genera of Actinobacteria are Streptomyces (nearly 70%), Nocardia, and Micromonospora, although Actinoplanes, Micromonospora, and Streptosporangium also are generally found. At present, the molecular identification is predicated on 16S rDNA sequences, which is most vital for Actinobacteria (Table 1).
3. Actinomycetes—a biofactory of novel enzymes
Actinomycetes is a genus of bacteria belonging to the Actinobacteria class. They’re all gram-positive bacteria. Actinomycetes species are facultatively anaerobes (with the exception of A.meyeri and A. israelii, which are both obligate anaerobes), and they thrive in anaerobic environments. Individual bacteria of Actinomyces species can generate endospores, and colonies of Actinomyces develop fungus-like branching networks of hyphae. The appearance of these colonies led to the false belief that the creature was a fungus, and the name Actinomyces, which means “ray fungus,” was given to it (from Greek actis, ray, beam and myles, fungus). Actinomycetes species may be found in soil as well as in animal microbiota, including the human microbiome. They are well-known for their importance in soil ecology; they generate a variety of enzymes that aid in the decomposition of organic plant material, lignin, and chitin. As a result, their presence is critical in the composting process. Humans and cattle have commensal flora on their skin, mouth flora, gut flora, and vaginal flora. They’re also renowned for causing infections in humans and cattle, mainly by gaining entrance to the inside of the body through wounds. People with immunodeficiency are more susceptible to opportunistic infections, as they are to other opportunistic illnesses. They are comparable to Nocardia in all of the qualities listed above, as well as in their branching filament production. Actinomycetes species, like many other anaerobes, are finicky and difficult to cultivate and isolate. Although clinical laboratories culture and isolate them, a negative result does not rule out infection since unwillingness to grow in vitro might be the cause.
4. Role of actinomycetes as environmental cleaner
Actinomycetes work as an environmental scrubber for the cleaning of the contaminant place and release potential enzymes for the same. The widely used application of Actinomyectes is shown in Figure 2.
Actinomycetes are well-known for their bioremediation abilities. Antibiotics and chemical complexes are effectively consumed by actinomycetes. Pesticides and chemical complexes at large dosages can be degraded by them. Petroleum hydrocarbons are widely employed as chemical components and fuel in our daily lives. Petroleum has become one of the most prevalent pollutants of large soil surfaces as a result of increased use, and is now regarded a serious environmental hazard. In the environment, hydrocarbons degrade in a variety of ways. Bioremediation is one of the methods for removing them from the environment.
The utilization of soil organisms to breakdown contaminants into innocuous chemicals is known as bioremediation. Pesticides and other xenobiotics in the environment are successfully disintegrated and bioremediated by the actinomycetes. Actinomycetes play an essential role in the environmental destiny of hazardous metals, altering transitions between soluble and insoluble phases through a variety of physico-chemical and biological processes, and producing considerable quantities of biosurfactants. These mechanisms are key components of natural biogeochemical cycles for metals and metalloids, and some of them might be used to remediate polluted materials. Actinomycetes’ role in bioremediation and stress-related behavior has been well investigated. Numerous Actinomycetes strains from composts are presently being studied to see if they can breakdown certain petroleum hydrocarbons and decolorize several synthetic dyes, which might lead to bioremediation applications. Actinomycetes have a number of characteristics that make them ideal candidates for bioremediation of organically polluted soils. They are capable of degrading complex polymers and play a vital role in the recycling of organic carbon. According to certain research, Streptomyces flora may play a critical role in hydrocarbon breakdown. Many strains produce cellulose- and hemicellulose cellulose degrading enzymes as well as extracellular peroxidase, which may solubilize lignin and destroy lignin-related compounds. Actinomycetes are the leading category of degraders in some polluted locations . Actinomycetes have the capacity to survive in an oily environment. As a result, these bacteria can be used in Bioremediation to remove oil contaminants.
4.2 Actinomycetes to subtract environmental oil pollutants
A dark sticky naturally occurring liquid (petroleum) that is called as Crude oil, a complex mixture of compounds having varying molecular weight and containing 30% polyaromatic hydrocarbons (PAHs). PAH compounds such as naphthalene, acenaphthene, fluorene, phenenthrene, fluoranthene, pyrene, acenaphthylene, benz[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, dibenz[a, h]anthracene, benzo[g, h, i]perylene, anthracene, and indeno[1,2,3cd] pyrene) are pollutants nominated by the United States Environmental Protection Agency as priority PAHs. The most widespread organic pollutants and potentially health hazards are targeted for measurement in environmental samples. On the other hand it is also found in environmental components like cereals, oils, fats, vegetables, cooked that are carcinogenic, mutagenic, and teratogenic. So removal of PAHs is an issue of big interest . Crude oil containing various PAHs, focuses on metabolic pathways for its degradation and microbial degraders. In aerobic or anaerobic conditions, bacterial and fungal strains are able to target the specific PAH through effective and eco-friendly bioremediation approach currently. Additionally, a new approach will be needed to design for dearomatization of crude oil to shoot a solution in numerous PAH inhabitants.
The nocardioform actinomycetes of the genera
A novel anthracene degrading actinomycete was isolated from a hydrocarbon contaminated soil at mechanical engineering workshop. Haloalkalitolerant actinomycetes, Kocuria rosea, Kocuria palustris, Microbacterium testaceum, and Nocardia farcinica were used in investigation of the correct fluorescence method to check the PAHs biodegrading capacity of actinobacteria. In the fluorescence method, excitation and emission fluorescence were used in study of the PAHs biodegrading to determine the residual anthracene concentration .
A glass bead system was developed for growth of PAH-degrading actinomycetes in liquid culture. Here, R
Biodesulfurization was a selective and cost-effective method for subordinating the sulfur content of petroleum products. DBT, used as a model polyaromatic sulfur heterocycle for microbial isolation and characterization to check capability of transforming organosulfur compounds found in an assortment of fossil fuels. However, Biotransformations were occurred through metabolic degradative pathways or growing with it as a sole sulfur source and biocatalytic desulfurization for the selective removal of polyaromatic sulfur heterocycle.
The sulfur oxides released after incineration of fossil fuels, the main environmental problem, cause air pollution and are the key for acid rain. Organosulfur compounds, including dibenzothiophene (DBT) are metabolized by microbes.
An application of biosurfactants while one needs to clean up oil. Arthrofactin, a novel biosurfactant was produced by Arthrobacter species strain MIS38. Arthrofactin is one of the most effective lipopeptide biosurfactants and it effectively removes oil .
4.3 Actinomycetes in pesticide degradation
India, an agricultural country, has lost 30% of agricultural produce to pests. As a result insecticides, fungicides, pesticides and herbicides have rising demand for protection of crops. Monocrotophos (MCP) is organophosphorus pesticide and hazardous and extensively utilized in India to protect economically important crops. Biomineralization of Monocrotophos, by soil bacteria
A pollutant 4-chlorophenol (4-CP), toxic and recalcitrant compound which is formed from chlorination of waste water, in pulp mills, from breakdown of herbicides such as 2,4-dichlorophenoxyacetic acid and from anaerobic degradation of more highly chlorinated phenols, such as pentachlorophenol and 2,4,6-trichlorophenol. In Bioremediation, certain microbial strains are able to degrade 4-CP. A novel strains Arthrobacter chlorophenolicus sp. capable of degrading up to 350 p.p.m. (2.7 mM) high concentration of 4-CP (Westerberg et al., 2000). Arthrohacter ureafaciens CPR706 degrades 4-chlorophenol through the hydroquinone pathway .
Triazine rings are found in pesticides, plastic resins, dye s-Triazine herbicides are broadly used in modern agriculture, where they kill susceptible plants by coordinating to the quinone-binding protein in photosystem II, thereby inhibiting photosynthetic electron transfer. Atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-s-triazine) is broadly used herbicides in the United States for the control of broadleaf weeds in corn, sorghum, and sugarcane.
The chloroacetanilides including alachlor [2- chloro-2′,6′-diethyl-N-(methoxymethyl) acetanilide] are selective herbicides extensively used for pre-emergent weed control. The extremely toxic alachlor (144 mg l−1 concentration) was biologically degraded enzymatically by the metabolic pathway of the strains
4.4 Actinomycetes involved in plastic degradation
Plastics, corrosion resistant materials, strong, durable and inexpensive polymers are derived from petrochemicals and chemical processes to produce long chain polymers. At room temperatures the plastic polymers are not considered as toxic, but when heat is released from plastics have undesirable effects on the environment and human health. Plastics are accessible in a variety of forms such as nylon, polycarbonate, polyethyleneterephthalate, polyvinylidene chloride, Urea formaldehyde, polyamides, polyethylene, polypropylene, polystyrene, polytetraflouro ethylene, polyurethane and polyvinyl chloride. Naturally occurring microorganism involved in degradation of organic material- plastic and plastic waste. Styrene is possibly carcinogenic to humans that cause mammary gland tumors in animals . Polystyrene waste accumulates in the environment posing an increasing ecological threat .
The plastics of poly (β-hydroxybutyrate)(PHB)-and poly (ε-caprolactone)(PCL) were degraded by aerobic microorganisms that persist in the natural environment. The plastic depolymerizing microorganisms are distributed over many kinds of material, including landfill leachate, compost, sewage sludge, forest soil, farm soil, paddy soil, weed field soil, roadside sand, and pond sediment . Actinomycete strains
Biofilms are the favored bacterial mode of living and survival, most microorganisms—which tend to attach to surfaces—to gain physical support, increase nutrient utilization. Polyethylene (PE), synthetic polymer, is highly inert and virtually non-biodegradable.
Polylactic acid (PLA), biodegradable plastic has broadly applicable in food packaging with respect to environmental concern in solid-waste management. Novel polylactic acid-packaging degrading actinomycete, Streptomyces sp. KKU215 biomass productions were carried out in PLA-packaging as sole carbon source. The potent strain was used in biodegradation of PLA-packaging . Amycolatopsis strains, poly(L- lactide) degrader stain has ability to assimilate degradation product like poly lactic acids. .
4.5 Actinomycetes as biological degrader of dyes
Synthetic dyes, coloring agents are mostly used in textile industries and spawn a huge amount of wastewater during the process of dyeing. The release of colored effluents in rivers and lakes are the key reason for reduction of dissolved oxygen concentration creating anoxic condition and foremost to the acute toxic effects on the flora and fauna of the ecosystem. In addition to colored effluents in water bodies reduces the photosynthesis as it hampers dispersion of light in water. The color of textile wastewater deduction is a major environmental concern . These synthetic dyes are not easily removed in waste water treatment plants .
Azo dyes, water-soluble reactive dyes constitute the most versatile class of synthetic dyes used in the textile, pharmaceutical, paper, food and cosmetic industry due to their ease in production and variety in color compared to natural dyes. Azo dyes are widely used in textile industries. When Azo dyes are left in water bodies without any treatment, they result in environmental pollution and in turn are toxic, carcinogenic and mutagenic. Azo dye Reactive Yellow Biodegradation was carried out by microorganisms isolated from the activated sludge. The isolated actinomycetes were acclimatized to different concentrations of dye from 0.005–0.200% (mg/100 ml). The consortium was developed by mixing five actinomycetes and the found degradation of dye depends on the concentration of dye in addition to the growth of the actinomycetes. Lignin peroxidase, laccase and tyrosinase enzymes were responsible for steady degradation activities. Biosorption of Reactive Yellow dye as occurred by using dead biomass of actinomycetes . The
Reactive dye is the foremostly used one type of azo dye containing different reactive groups. Reactive dyes that derive from dyeing industries increase Chemical Oxygen Demand (COD) and Biological Oxygen Demand (BOD), change the pH of water bodies and it causes serious problems in plant, animal and human beings. The presence of dyes in water is highly visible and affects their transparency and esthetic even though the concentration of the dyes is low . Actinomycete strains were decolorized effluents containing different types of reactive dyes anthraquinone, phthalocyanine and azo dyes. The absorption of reactive dyes was done by the strains cells outcome in the decolorization of the effluents . Actinomycete
4.6 Actinomycetes and heavy metal
Heavy metals were successfully removed from wastewaters, and industrial wastes are still a key study area today. Streptomyces coelicolor’s application in heavy metal removal via interactions is consistent with traditional heavy metal responses, resistance mechanisms, and secondary metabolite formation. Some physiological features of the salt sensitive cultivar Giza 122 of
4.7 Actinomycetes in removal of groundwater pollutant
Pollution of the environment is becoming more of a global issue. This circumstance necessitates rapid technological adaptation. Biological remediation, which involves the use of live organisms or their products, is a viable option. Actinobacteria have proven to be an effective instrument for carrying out this procedure on various matrices, under various growth conditions, as pure cultures or consortia, or as enzyme and emulsifier providers. The next stage is to use this information to a larger scale in the field. Actinomycetes strains can be identified and used for solid waste biodegradation with success. The consortium of selected actinomycetes strains might be used to assess their economic viability in the biodegradation process. This broadens their applications in biotechnology and environmental research.