Actinobacteria- An Overview

Actinobacteria are Gram-positive, aerobic or facultative, spore-forming bacteria belonging to the order Actinomycetales, characterized by high guanine and cytosine (G + C) content (>55 mol %) in DNAs and rod-shape or mycelium like filamentous structures.

The name “Actinomycetes” was derived from the Greek words “atkis” (a ray) and “mykes” (fungus). This suggests their structural similarity with fungi, but they are biochemically more close to bacteria and hence are included in the domain of Bacteria. They are connecting links between “Fungus” and “Bacterium”. They are also referred to as Actinobacteria or Actinomycetota.

Actinobacteria is not a genus of bacteria, rather it is a “Phylum” inclosing a large number of genera (more than 425 genera are known), each with multiple species. 

It is a heterogeneous group including bacteria with distinctive habitats, growth requirements, ecological roles, morphology, and applications. In fact, they are among the most important and widely used bacteria for medical, agricultural, environmental, biotechnological, and research purposes.  

General Characteristics of Actinobacteria

• Gram-positive, branched filamentous, or rod-shaped
• Reproduction via spore and fragmentation
• Fungus-like aerial mycelium and substrate mycelium production in culture
• Pigmentation in most
• High G + C content in DNA (55 – 75%)
• Ubiquitous in habitat (symbiotic, free-living, varied temperature, pH, salinity, oxygen)
• Mucolic acid and muramic acid are found in cell walls and filaments
• Relatively longer generation time than other bacteria
• Most are non-motile
• Most are non-capsulated

Actinobacteria Classification

Due to diverse physical and chemical structures and new findings of evolving genetic analysis systems, the Actinobacteria classification system is not rigid. From time to time, the systematics of Actinobacteria is changing. The most commonly accepted classification scheme is included in this description.

Domain: Bacteria

Phylum: Actinobacteria

In Bergey’s Manual of Systematic Bacteriology Volume 5, the phylum Actinobacteria is classified into 6 classes. They are:

1. Actinobacteria (now called Actinomycetia) 
2. Acidimicrobiia
3. Coriobacteriia
4. Nitriliruptoria
5. Rubrobacteria
6. Thermoleophilia

Some Common Actinobacteria Genera

Habitat and Ecology of Actinobacteria

Actinobacteria are ubiquitous in habitats with high versatility in nutrient requirement, biotic and abiotic association, and physiochemical requirements like pH, humidity, salinity, temperature, air, etc. 

They are found in terrestrial as well as aquatic habitats.

• Terrestrial habitat includes almost every type of soil like a desert (sandy), rocky, agricultural, vegetation-rich, fertile, and barren soil. Actinobacteria cover most of the soil microbiome with a microbial load of about 10⁶ – 10⁸ cells per gram of soil. (The earthy smell of freshly plowed fields is also due to the action of geosmin and 2-methylisoborbeol produced by Actinobacteria). Soil actinobacteria are the most studied ones and are very important in biogeochemical processes as well as human applications. 
  Some common soil actinobacteria are Streptomyces, Micromonospora, Nocardia, Pseudonocardia, Actinomadura, Mycobacterium, etc. 
• Aquatic habitat includes both marine and freshwater. Even extremely cold water (artic water and snowy places) and extremely hot (hydrothermal vent) have some genera of actinobacteria. 
  Some common freshwater genera are Streptomyces, Rhodococcus, Actinoplanes, Thermoactinomyces, Micromonospora, Corynebacterium, Arthrobacter, etc. 
  Some common marine genera are Rhodococcus, Salinispora, Streptomyces, Marinophilus, Salinibacterium, Dietzia, Solwaraspora, etc. 

• Several species of Actinobacteria are extremophiles. They are found in freezing temperature (psychrophile), normal temperature (mesophile), as well as high temperature (thermophile). Corynebacterium psychrophilum, Modestobacter multiseptatus, Streptoverticillium spp. etc are psychrophilic species, whereas Thermoactinomyces, Thermomonospora, Saccharopolyspora, etc. are thermophilic species.   
• Some are found in pH around 3 (acidophilic actinobacteria); like Streptoacidiphilus, Actinospica, Catenulispora, Streptomyces acidiphilus, etc. are acidophiles. 
• Few species like Bogoriella caseilytica, and Kocuria spp., are halo-alkalophilic Actinobacteria. 
• Several species are found in soil with high salt concentration (some even require nearly 5M NaCl concentration in media to grow). Some halophilic Actinobacteria are Salinispora, Dietzia, Williamsia, Marinophilus, etc. 
• Endophytic Actinobacteria are also widely isolated and studied. Most of them are associated with the rhizosphere and are associated with nitrogen fixation. Frankia spp., Streptomyces spp., Streptoverticillium spp., Glycomyces spp., Plantactinospora spp., Polymorphospora spp., etc are endophytic actinobacteria. 
• Numerous genera are symbiotic. They are associated with plants, animals, and several other organisms. Several of them are normal habitant of the gut, skin, and respiratory tract.    

Morphology of Actinobacteria

• The diverse morphological feature is seen among the members of Actinobacteria. They display different cellular arrangements and branching. 
• In general, they are rod-shaped, about 0.5 to 2.5 μm (some are even up to 5 μm) in length. Some like Micrococcus are cocci also. 
• Many species produce hyphal (mycelial) growth. These mycelia are also of different shapes. Some are branched, some twisted, some straight, some are septed, and some asepted. 
• Aerial mycelium, bearing spore or spore-forming structure, is a key feature in identifying several species. These aerial mycelia may be straight, flexuous, fascicled, monoverticillate without spirals, monoverticillate with spirals, open loops, open spirals, closed spirals, and biverticillate with and without spirals. 
• Three different types of spores are observed viz. conidiospores, sporangiospores, and oidiospores. 
• Few species contain fimbriae, while several lack them. Hence, most of them are non-motile. 
• On culture, most of them show dry pigmented colonies with aerial and substrate hyphae giving a cottony, velvety, or powdery appearance.  Pigmentation is seen in both aerial and substrate hyphae. Few (capsulated species) can also produce moist colonies. 

Actinobacteria Identification Method

• The most precise identification method currently in use is 16S rRNA analysis. Besides, the PCR method and DNA probe method are also crucial molecular identification methods.
• The microscopic study of cellular structure, spore-forming structure, spores, branching pattern, and structure of filaments is an essential feature for identification. In addition to these features, colony morphology, pigmentation, and biochemical characteristics are employed to identify the isolated species. 
• International Streptomyces Project (ISP) media (ISP media from ISP 1 to ISP 9) are commonly used media for the growth and study of colony characters.
• Standard biochemical tests performed are Gram-staining, catalase test, salt tolerance test, oxidase test, nitrate reduction test, starch hydrolysis test, casein hydrolysis test, gelatin hydrolysis test, urea hydrolysis test, IMViC test, TSI test, sugar fermentation test, etc.  

Significance of Actinobacteria 

Roles in Nature

• They are significant decomposers in nature. Degradation of complex organic matters like cellulose, lignin, keratin, chitin, hemicellulose, and other complex polymers depends on Actinobacteria. This degradation aids in soil fertility and the promotion of vegetation.   
• They play a major role in nutrient cycling in nature. They are associated with the decomposition of organic matter and assimilation of inorganic matter; hence play an important role in biogeochemical cycles. 
• They are found in extreme environments and maintain the biogeochemical cycle in such harsh niches. They help sustain life forms in such harsh conditions.   
• They release several enzymes, vitamins, proteins, antimicrobials, and micronutrients that affect the soil and aquatic habitat microbial flora.    

Human Applications of Actinobacteria

Pharmaceutical Uses

• Actinobacteria are the significant source of antimicrobials. More than 2/3^rd of total antibiotics used are derived from several Actinobacteria, especially from Streptomyces spp. and Micromonospora spp.  
• Some common Actinobacteria derived antibiotics are tetracyclines, cephamycins, macrolides (like streptomycin, erythromycins, gentamicin, etc.) quinolones, chloramphenicol, some aminoglycosides, some β- lactams, rapamycin, etc.
• Antifungal compounds like amphotericin, daryamides, etc., an antiparasitic compound like avermectin, and anticancer compounds like Adriamycin, 1,8-Dihydroxy-2-ethyl-3 methyl anthraquinone, anthracyclines, butenolides, etc. are also derived from Actinobacteria. 

Bioremediation and Composting

Actinobacteria are widely used in the bioremediation of soil polluted with recalcitrant organic matter. They are dominant in soil rich in complex organic matter and hydrocarbons. 

During compost preparation, mesophilic Actinobacteria and other saprophytes initially act on organic wastes, and as the temperature crosses 40°C, thermophilic Actinobacteria dominate other saprophytes. And finally, during the maturation phase of composting, fungal species and Actinobacteria acting on celluloses, hemicelluloses, chitin, keratin, and other complex compounds dominate. 

Enzymes and Vitamins Production

Several enzymes with potential application in the fermentation and food industry, textile and fabric industry, leather industry, paper, and pulp industry, and other biotechnological applications are produced by Actinobacteria. Enzymes produced commercially are chitinases, catalases, L-asparaginase, proteases, ureases, keratinases, lipase, amylase, cellulases, xylanases, and phytases produced by several Actinobacteria.  Streptomyces spp., Nocardiopsis spp., Thermobifida spp., Actinomadura spp., etc. are used to produce such enzymes. 

Vitamin-B₁₂ is produced by soil Actinobacteria like Streptomyces antibioticus, S. chromogenus, etc. 

Biosurfactant Production

Different species of Actinobacteria like Nocardiopsis spp., Tsukamurella spp., etc produce several surfactants. These biosurfactants contain a hydrophilic and a hydrophobic moiety. They are used for the emulsification of oil and hydrocarbons in bioremediation, for the production of detergents, herbicides, cosmetics, antimicrobials, etc., and used in paper, pulp, textile, and food industries.  

Bioherbicides, Bioinsecticide, and Biolarvicide Production

Several secondary metabolites released by several Actinobacteria genera are used as bioherbicides to control unwanted weeds. For instance, Streptomyces saganonensis, S. viridochromogenes, and several other Streptomyces spp. produce herbicidines and herbimycins to control weed growth. Anisomycin, bialaphos, coformycin, hydantocidin, phthoxazolin, homoalanosin, etc. are herbicidal compounds produced by several Streptomyces spp.

Insecticidal and larvicidal compounds are found to be produced by Streptomyces spp., Micropolyspora spp., and Streptosporangium spp. Secondary metabolites like tetranectin, avermectins, flavonoids, etc. are active against mosquitoes.    

Plant Growth Hormones Production

Several soil Actinobacteria associated with rhizomes are identified as Plant Growth Promoting Rhizobacteria (PGPR). They help in nutrition assimilation and acquisition, produce plant growth-promoting hormones, and produce antimicrobials that fight against plant pathogens.  

Plant Diseases and Plant Parasite Controlling Agents

Avermectin, and Ivermectin, produced by Streptomyces spp. are found to show nematicidal effects on soil netamodes and filarial worms. Besides, Salinispora spp., Marinactinospora spp., etc. are also found to produce antiparasitic compounds. 

Agroactive compounds like antibiotics and antifungal against plant pathogens such as Rhizoctonia spp., Pseudomonas spp., Pyricularia oryzae, etc. are produced by Actinobacteria like Streptomyces spp., Micromonospora spp., Streptoverticillium spp., etc. Compounds like geldanamycin, nigericin, faerifungin, validamycin, fungichromin, etc. are produced by Actinobacteria that are used to cure plant diseases.     

Nanoparticle Synthesis

Several Actinobacteria are essential sources for the production of nanoparticles having pharmaceutical properties. Important species producing silver nanoparticles are Streptomyces spp., Nocardiopsis spp., Actinopolyspora spp., Actinomadura spp., etc. Streptomyces spp., are also found to produce gold, zinc, copper, and magnesium nanoparticles. 

Pigment Production

Actinobacteria are capable of producing various pigments, usually shades of blue, violet, red, yellow, pink, green, black, and brown.  Though most of the industrially used pigments are chemically produced, Actinobacteria like Streptomyces spp., and Synodontis violaccus are used in the commercial production of a few pigments like rhodomycin, actinomycin, granaticin, and a few prodigiosin. 

Harmful Effects

Plant Diseases

Actinobacteria are responsible for various plant diseases. Some of them are tabulated below:

Human Diseases

Most Actinobacteria are saprophytes and commensals, but few are associated with human disease also. They are responsible for deadly diseases like Tuberculosis, Leprosy, Diphtheria, actinomycosis, nocardiosis, etc. Some common human pathogenic Actinobacteria species along with diseases caused by them are listed below:

References

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