Actinobacteria- An Overview

Actinobacteria are a group of bacteria that belong to the phylum Actinobacteria, which is one of the largest and most diverse phyla in the domain of Bacteria. They are Gram-positive bacteria with high guanine and cytosine (G+C) content in their DNA, ranging from 55% to 75%. They can be terrestrial or aquatic, and they can be found in various habitats, such as soil, water, plants, animals, and humans. They have diverse morphological features, such as rod-shaped, coccoid, filamentous, or branched forms. Some of them produce spores and mycelia, which are fungal-like structures that help them survive in harsh environments. They also produce pigments that give them different colors, such as blue, red, yellow, green, or brown.

Actinobacteria are important for many ecological and biotechnological processes. They play a key role in the decomposition of organic matter and the cycling of nutrients in nature. They also produce a wide range of bioactive compounds, such as antibiotics, antifungals, anticancer agents, enzymes, vitamins, hormones, and surfactants. Many of these compounds have been used for medical, agricultural, industrial, and research purposes. Some examples of well-known actinobacterial genera that produce useful compounds are Streptomyces, Micromonospora, Nocardia, Actinomyces, Frankia, and Mycobacterium.

However, not all actinobacteria are beneficial. Some of them can cause diseases in plants and animals, including humans. For instance, Mycobacterium tuberculosis causes tuberculosis, Corynebacterium diphtheriae causes diphtheria, and Actinomyces israelii causes actinomycosis. These pathogens can be difficult to treat because of their resistance to many antibiotics.

In this article, we will provide an overview of the general characteristics, classification, common genera, habitat and ecology, morphology, identification methods, significance, and harmful effects of actinobacteria. We will also discuss some of the current challenges and future perspectives of actinobacterial research.

Actinobacteria are a group of Gram-positive bacteria that have a high guanine and cytosine (G + C) content in their DNA, ranging from 55% to 75%. They are also known as actinomycetes, which means "ray fungi" in Greek because they produce branching filaments that resemble fungal hyphae. However, they are not closely related to fungi and belong to a different domain of life.

Some of the general characteristics of actinobacteria are:

• They can be rod-shaped or filamentous, and some form spores for reproduction or survival. The spores can be borne on aerial hyphae (called conidia) or enclosed in sac-like structures (called sporangia).

• They can produce pigments that give them various colors, such as blue, red, yellow, green, brown, or black. The pigments can be found in the aerial hyphae, the substrate hyphae, or the spores.

• They are mostly aerobic or facultatively anaerobic, meaning they can use oxygen or other electron acceptors for respiration. Some are obligate aerobes that require oxygen, while some are obligate anaerobes that cannot tolerate oxygen.

• They are ubiquitous in nature and can inhabit diverse environments, such as soil, water, plants, animals, and humans. They can adapt to various temperature, pH, salinity, and oxygen levels. Some are extremophiles that can live in harsh conditions, such as high or low temperatures, acidity, alkalinity, or salinity.

• They have a complex cell wall structure that contains peptidoglycan, arabinogalactan, mycolic acid, and muramic acid. The cell wall provides rigidity and protection to the cells and also contributes to their Gram-positive staining.

• They have a relatively long generation time compared to other bacteria, which means they grow and divide slowly. This may be related to their high G + C content, which makes their DNA more stable but also more difficult to replicate.

• They are mostly non-motile and non-capsulated, meaning they do not have flagella or other structures for movement or capsules for attachment. However, some exceptions exist, such as Corynebacterium diphtheriae, which has fimbriae for adhesion.

Actinobacteria are important for their ecological roles as decomposers and nutrient cyclers in nature. They can degrade complex organic compounds such as cellulose, lignin, chitin, keratin, and hemicellulose. They also produce enzymes, vitamins, proteins, antimicrobials, and micronutrients that affect the soil and aquatic microbial communities.

Actinobacteria are also valuable for their biotechnological applications as producers of secondary metabolites. They are the source of more than two-thirds of the antibiotics used in medicine and agriculture. They also produce antifungal, antiparasitic, anticancer, immunosuppressive, herbicidal, insecticidal, larvicidal, biosurfactant, bioherbicide, and bioremediation agent compounds.

However, actinobacteria can also cause diseases in plants and animals. Some of them are pathogens that infect crops and cause losses in agriculture. Some of them are opportunistic pathogens that infect humans and animals with weakened immune systems. Some of the diseases caused by actinobacteria are tuberculosis (Mycobacterium tuberculosis), leprosy (Mycobacterium leprae), diphtheria (Corynebacterium diphtheriae), actinomycosis (Actinomyces spp.), nocardiosis (Nocardia spp.), rhodococcosis (Rhodococcus equi), etc.[^1 ^][ ^2 ^].

Actinobacteria are a phylum of Gram-positive bacteria with high G+C content in their DNA. They are diverse in their morphology, ecology, and metabolism, and include many important genera that produce antibiotics, fix nitrogen, decompose organic matter, and cause diseases.

The classification of Actinobacteria is not fixed and has changed over time due to new discoveries and advances in molecular methods. The most commonly accepted classification scheme is based on 16S rRNA gene sequences, which are used to infer the evolutionary relationships among different groups of bacteria. However, other molecular markers, such as RpoB, SecY, RecA, GyrB, DnaK, and GrpE, have also been used to construct phylogenetic trees of particular groups of Actinobacteria, such as Mycobacterium, Bifidobacterium, and Streptomyces. In addition, whole genome sequences have been employed for taxonomy studies of Actinobacteria.

According to the latest classification by Goodfellow (2021), the phylum Actinobacteria consists of six classes:

• Actinomycetia (formerly Actinobacteria), which includes the order Actinomycetales and its suborders. This class contains many well-known genera of Actinobacteria, such as Streptomyces, Nocardia, Micromonospora, Actinomyces, Frankia, and Mycobacterium.

• Acidimicrobiia, which includes the order of Acidimicrobiales and its suborders. This class contains mostly acidophilic bacteria that are found in acidic soils and aquatic environments.

• Coriobacteriia, which includes the order of Coriobacteriales and its suborders. This class contains mostly anaerobic bacteria that are associated with the gut microbiota of animals and humans.

• Nitriliruptoria, which includes the order Nitriliruptorales and its suborders. This class contains mostly aerobic bacteria that can degrade nitriles and cyanides.

• Rubrobacteria, which includes the order Rubrobacterales and its suborders. This class contains mostly thermophilic bacteria that are found in hot springs and geothermal soils.

• Thermoleophilia, which includes the order Thermoleophilales and its suborders. This class contains mostly thermophilic bacteria that are found in composts and other high-temperature environments.

The six classes of Actinobacteria are further divided into subclasses, orders, suborders, families, and genera based on their phenotypic and genotypic characteristics. The current taxonomy of Actinobacteria is summarized in Table 1.

Table 1: Taxonomy of Actinobacteria (adapted from Goodfellow 2021)

| Class | Subclass | Order | Suborder | Family | Example genera |

| --- | --- | --- | --- | --- | --- |

| Actinomycetia | Acidimicrobidae | Acidimicrobiales | Acidimicrobineae | Acidimicrobiaceae | Acidimicrobium |

| | | | Rubrobacterineae | Rubrobacteraceae | Rubrobacter |

| | | | Solirubrobacterineae | Conexibacteraceae | Conexibacter |

| | | | | Patulibacteraceae | Patulibacter |

| | | | | Solirubrobacteraceae | Solirubrobacter |

| | Coriobacteridae | Coriobacteriales | Coriobacterineae | Atopobiaceae | Atopobium |

| | | | | Coriobacteriaceae | Coriobacterium |

| | | | Eggerthellineae | Eggerthellaceae

Actinobacteria is a large and diverse phylum of Gram-positive bacteria that have a high GC content in their DNA. They are widely distributed in various habitats, such as soil, water, plants, animals, and humans. Some of them are beneficial for agriculture, biotechnology, and medicine, while others are pathogenic or opportunistic.

There are hundreds of genera of Actinobacteria, but some of the most common and important ones are:

• Streptomyces: This is the largest and most studied genus of Actinobacteria, comprising over 900 species. They are soil-dwelling bacteria that produce aerial hyphae and spores. They are also the major source of natural antibiotics, such as streptomycin, tetracycline, erythromycin, and chloramphenicol. They also produce other bioactive compounds, such as antifungals, antivirals, anticancer agents, and immunosuppressants.

• Mycobacterium: This genus includes some of the most notorious human pathogens, such as Mycobacterium tuberculosis (the causative agent of tuberculosis), Mycobacterium leprae (the causative agent of leprosy) and Mycobacterium ulcerans (the causative agent of Buruli ulcer). They are acid-fast bacteria that have a thick and waxy cell wall that makes them resistant to many antibiotics and disinfectants.

• Corynebacterium: This genus includes both harmless and pathogenic species. The most important pathogen is Corynebacterium diphtheriae, which causes diphtheria, a respiratory infection that can be fatal if not treated with antitoxin and antibiotics. Other species can cause skin infections, urinary tract infections, and endocarditis.

• Nocardia: This genus includes aerobic bacteria that form branching filaments and can cause nocardiosis, a chronic infection that mainly affects the lungs, skin, and central nervous system. Nocardiosis is more common in immunocompromised individuals, such as those with HIV/AIDS or organ transplant recipients. Some species can also degrade hydrocarbons and pesticides.

• Actinomyces: This genus includes anaerobic or microaerophilic bacteria that form colonies resembling fungi. They are part of the normal flora of the oral cavity, gastrointestinal tract, and female genital tract. However, they can cause actinomycosis, a chronic infection that forms abscesses and fistulas in various organs. Actinomycosis is often associated with poor oral hygiene or dental procedures.

• Frankia: This genus includes nitrogen-fixing bacteria that form symbiotic relationships with certain plants, such as alder, casuarina, and bayberry. They form root nodules, where they convert atmospheric nitrogen into ammonia that can be used by the plant for growth. They also produce siderophores that help the plant acquire iron from the soil.

• Micromonospora: This genus includes soil bacteria that produce spores and have filamentous growth. They are also prolific producers of antibiotics, such as gentamicin, rifamycin, and neomycin. They also produce enzymes that can degrade chitin, cellulose, and lignin.

• Rhodococcus: This genus includes aerobic bacteria that have a coccoid or rod-shaped morphology. They are widely distributed in soil, water, and air. Some species can degrade various organic pollutants, such as naphthalene, toluene, and phenol. Some species can also cause infections in animals and humans, such as Rhodococcus equi, which causes pneumonia in foals and immunocompromised people.

These are some of the common genera of Actinobacteria, but there are many more that have diverse characteristics and roles in nature and human health. Actinobacteria are an important group of bacteria that deserve more attention and research.

Actinobacteria are one of the most diverse and widespread groups of bacteria in nature. They can adapt to various ecological environments, including soil, freshwater, saltwater, aerial, extreme, and symbiotic habitats. They play important roles in biogeochemical cycles, the decomposition of organic matter, the production of bioactive compounds, and interactions with other organisms.

Soil Habitat

Soil is the most common and rich habitat for actinobacteria, especially for the order Actinomycetales, which are filamentous and spore-forming bacteria. Actinobacteria can be found in almost every type of soil, such as desert, rocky, agricultural, vegetated, fertile, and barren soils. They can also tolerate different physicochemical conditions, such as pH, humidity, salinity, temperature, and oxygen levels. Actinobacteria account for about 10-20% of the total bacterial population in the soil, with a density of 10^6 to 10^8 cells per gram of soil.

Actinobacteria are important decomposers in soil, as they can degrade complex organic polymers, such as cellulose, lignin, keratin, chitin, and hemicellulose. They also participate in nutrient cycling, such as nitrogen fixation, nitrification, and denitrification. Some actinobacteria produce antibiotics, enzymes, vitamins, and other metabolites that affect the soil microbial community and plant growth. For example, Streptomyces spp., Micromonospora spp., Nocardia spp., Pseudonocardia spp., Actinomadura spp., Mycobacterium spp. are some common soil actinobacteria that have been isolated and studied for their bioactive potential.

Aquatic Habitat

Actinobacteria are also abundant and diverse in aquatic habitats, both marine and freshwater. They can be found in surface water, deep sea water, sediments, hydrothermal vents, coral reefs, and other aquatic ecosystems. They can also survive in extreme conditions, such as low or high temperature, high pressure, low oxygen, and high salinity. Actinobacteria contribute to the carbon and nitrogen cycles in aquatic environments, as well as the degradation of organic pollutants and xenobiotics. Some actinobacteria produce antimicrobial, antifungal, antiparasitic, and anticancer compounds that have potential applications in biotechnology and medicine. For example, Rhodococcus spp., Salinispora spp., Streptomyces spp., Marinophilus spp., Salinibacterium spp., Dietzia spp., Solwaraspora spp. are some common aquatic actinobacteria that have been isolated and screened for their bioactive properties.

Aerial Habitat

Actinobacteria are also present in aerial habitats, such as air, dust, and aerosols. They can be dispersed by wind, rain, or human activities from soil or water sources. They can also form biofilms or associations with other microorganisms or particles in the air. Actinobacteria can affect air quality and human health by producing volatile organic compounds (VOCs), allergens, or pathogens. Some actinobacteria produce geosmin and 2-methylisoborneol (MIB), which are responsible for the earthy smell of freshly plowed fields or rain. Some actinobacteria cause respiratory infections or diseases in humans or animals, such as tuberculosis (Mycobacterium tuberculosis), leprosy (Mycobacterium leprae), diphtheria (Corynebacterium diphtheriae), actinomycosis (Actinomyces spp.), nocardiosis (Nocardia spp.) and others.

Actinobacteria are also found in extreme habitats, where they can withstand harsh environmental conditions that limit the growth of most other microorganisms. They have developed various adaptations to cope with these challenges, such as modifying their cell wall structure, membrane composition, metabolic pathways, or gene expression. Some examples of extreme habitats where actinobacteria have been detected are:

• Freezing temperature (psychrophilic actinobacteria): They can grow at temperatures below 0°C or even below -20°C. They have mechanisms to prevent ice formation or damage to their cell membranes or enzymes. Some examples are Corynebacterium psychrophilum, Modestobacter multiseptate, Streptoverticillium spp..

• High temperature (thermophilic actinobacteria): They can grow at temperatures above 45°C or even above 80°C. They have mechanisms to stabilize their proteins or DNA against heat denaturation or degradation. Some examples are Thermoactinomyces spp., Thermomonospora spp., and Saccharopolyspora spp.

• Low pH (acidophilic actinobacteria): They can grow at a pH below 5 or even below 3. They have mechanisms to maintain their intracellular pH or protect their cell wall or enzymes from acid hydrolysis. Some examples are Streptoacidiphilus spp., Actinospica spp., Catenulispora spp., and Streptomyces acidophilus.

• High pH and salinity (haloalkaliphilic actinobacteria): They can grow at pH above 9 and salinity above 10% or even above 20%. They have mechanisms to regulate their osmotic pressure or ion balance or synthesize compatible solutes. Some examples are Bogoriella caseilytica and Kocuria spp.

• High salinity (halophilic actinobacteria): They can grow at salinity above 10% or even above 30%. They have mechanisms similar to haloalkaliphilic or produce specific pigments to cope with high salt stress. Some examples are Salinispora spp., Dietzia spp., Williamsia spp., and Marinophilus spp.

Symbiotic Habitat

Actinobacteria are also involved in symbiotic relationships with various plants and animals. They can form mutualistic or parasitic associations with their hosts, depending on the benefits or costs involved. They can provide nutrients, hormones, protection, or other services to their hosts or exploit their resources or cause diseases to them. Some examples of symbiotic habitats where actinobacteria have been found are:

• Plant roots (endophytic actinobacteria): They live inside plant tissues without causing any harm to them. They can help plants in nitrogen fixation, phosphate solubilization, growth promotion, or pathogen resistance. Some examples are Frankia spp., Streptomyces spp., Streptoverticillium spp., Glycomyces spp., Plantactinospora spp., and Polymorphospora spp.

• Animal guts (gut-associated actinobacteria): They live inside animal digestive tracts without causing any harm to them. They can help animals in digestion, fermentation, vitamin synthesis, or immune modulation. Some examples are Bifidobacterium spp., Collinsella spp., Eggerthella spp., and Gordonibacter spp.

• Animal skins (skin-associated actinobacteria): They live on animal skin surfaces without causing any harm to them. They can help animals in protection against pathogens or UV radiation or the production of odors or pheromones. Some examples are Propionibacterium acnes, Corynebacterium jeikeium, Micrococcus luteus, and Dermacoccus nishinomiyaensis.

• Insects (insect-associated actinobacteria): They live inside insect bodies or on insect surfaces without causing any harm to them. They can help insects with nutrition acquisition

Actinobacteria are a diverse group of gram-positive bacteria with high G+C content in their DNA. They exhibit various shapes, sizes, and arrangements of cells, as well as different types of hyphae and spores. Some of the common morphological features of Actinobacteria are:

• Most Actinobacteria are rod-shaped, ranging from 0.5 to 2.5 μm in length, but some are cocci (such as Micrococcus) or filamentous (such as Streptomyces).

• Many Actinobacteria produce hyphal growth, which can be divided into substrate mycelium and aerial mycelium. Substrate mycelium is the part that grows on or in the solid medium, while aerial mycelium is the part that projects above the surface and bears spores or spore-forming structures.

• The hyphae of Actinobacteria can be branched, twisted, straight, septate (with cross-walls), or aseptate (without cross-walls). The branching pattern and structure of the filaments can be used to identify different genera and species of Actinobacteria.

• Aerial mycelium can produce three types of spores: conidiospores, sporangiospores, and conidiospores. Conidiospores are single cells that are formed by fragmentation or budding of the hyphae. Sporangiospores are multiple cells that are enclosed in a sac-like structure called a sporangium. Oidiospores are single cells that are formed by fragmentation of the hyphae without any differentiation.

• Some Actinobacteria have fimbriae (hair-like appendages) on their cell surface, which may help them attach to surfaces or other cells. However, most Actinobacteria are non-motile and do not have flagella or pili.

• Most Actinobacteria do not have capsules (slime layers) around their cells, except for some pathogenic species (such as Mycobacterium tuberculosis) that use capsules to evade the host immune system.

• On culture media, Actinobacteria show different colors and textures of colonies, depending on their pigmentation and production of hyphae and spores. Some Actinobacteria produce soluble pigments that can diffuse into the medium, while others produce insoluble pigments that remain in the cells or hyphae. The colors can range from white, yellow, orange, red, pink, purple, blue, green, and brown to black.

The morphology of Actinobacteria is influenced by various environmental factors, such as temperature, pH, oxygen, nutrients, and stress conditions. Moreover, the morphology of Actinobacteria is also controlled by genetic factors, such as regulatory genes and signaling molecules that coordinate the development of hyphae and spores in response to starvation or other stimuli. Therefore, studying the morphology of Actinobacteria can provide insights into their ecology and physiology, as well as their potential for producing novel natural products.

Actinobacteria are a diverse group of bacteria that have a high GC content and produce a variety of bioactive natural products. They can be identified by different methods based on their morphology, physiology, chemistry, and genetics.

Morphological Methods

Morphological methods rely on the observation of the size, shape, color, and arrangement of the cells and spores of Actinobacteria. Some common morphological features that are used for identification are:

• Gram staining: Actinobacteria are Gram-positive bacteria, meaning they retain the crystal violet dye after decolorization with alcohol. However, some species may show variable or weak staining due to the presence of mycolic acids or other lipids in their cell walls.

• Acid-fast staining: Some Actinobacteria, especially those belonging to the genus Mycobacterium, are acid-fast, meaning they resist decolorization with acid-alcohol after staining with carbol fuchsin. This is due to the high content of mycolic acids in their cell walls, which makes them impermeable to the acid-alcohol solution.

• Pigment production: Many Actinobacteria produce pigments that give them distinctive colors on culture media or under the microscope. The pigments may be soluble or insoluble and may be produced in the aerial or substrate mycelium. The pigments may range from blue, violet, red, yellow, pink, green, black, and brown.

• Spore formation: Many Actinobacteria produce spores as a means of reproduction and survival under unfavorable conditions. The spores may be borne on aerial or substrate hyphae and may have different shapes and arrangements. Three types of spores are commonly observed: conidiospores, sporangiospores, and conidiospores. Conidiospores are formed by fragmentation of aerial hyphae into single cells or chains of cells. Sporangiospores are formed inside a sac-like structure called a sporangium at the tip of aerial hyphae. Oidiospores are formed by the budding of aerial hyphae into small spherical cells.

Physiological Methods

Physiological methods rely on the measurement of the metabolic activities and growth requirements of Actinobacteria. Some common physiological tests that are used for identification are:

• Catalase test: Catalase is an enzyme that breaks down hydrogen peroxide into water and oxygen. Most Actinobacteria are catalase-positive, meaning they produce bubbles when exposed to hydrogen peroxide. However, some species may be catalase-negative or weakly positive.

• Oxidase test: Oxidase is an enzyme that transfers electrons from a donor to oxygen, producing water or hydrogen peroxide. Most Actinobacteria are oxidase-negative, meaning they do not change the color of a reagent such as tetramethyl-p-phenylenediamine when exposed to oxygen. However, some species may be oxidase-positive or weakly positive.

• Nitrate reduction test: Nitrate reduction is a process that converts nitrate to nitrite or other nitrogenous compounds. Some Actinobacteria can reduce nitrate to nitrite using nitrate reductase enzyme. This can be detected by adding sulfanilic acid and alpha-naphthylamine reagents, which form a red color with nitrite. Some Actinobacteria can further reduce nitrite to ammonia or nitrogen gas using nitrite reductase or nitrate reductase enzymes. This can be detected by adding zinc dust, which reduces any remaining nitrate to nitrite and forms a red color if nitrate reduction is incomplete.

• Starch hydrolysis test: Starch hydrolysis is a process that breaks down starch into glucose using an amylase enzyme. Some Actinobacteria can hydrolyze starch using extracellular amylase enzyme. This can be detected by adding an iodine solution, which forms a blue-black color with starch but not with glucose.

• Casein hydrolysis test: Casein hydrolysis is a process that breaks down casein (a protein found in milk) into amino acids using protease enzyme. Some Actinobacteria can hydrolyze casein using extracellular protease enzyme. This can be detected by observing a clear zone around the bacterial growth on a milk agar plate.

• Gelatin hydrolysis test: Gelatin hydrolysis is a process that breaks down gelatin (a protein derived from collagen) into amino acids using the gelatinase enzyme. Some Actinobacteria can hydrolyze gelatin using extracellular gelatinase enzyme. This can be detected by observing the liquefaction of gelatin around the bacterial growth on a gelatin agar plate.

• Urea hydrolysis test: Urea hydrolysis is a process that breaks down urea (a nitrogenous waste product) into ammonia and carbon dioxide using the urease enzyme. Some Actinobacteria can hydrolyze urea using extracellular urease enzyme. This can be detected by observing a color change from yellow to pink on a urea agar plate due to the increase in pH caused by ammonia production.

Chemotaxonomic Methods

Chemotaxonomic methods rely on the analysis of the chemical composition and structure of the cell wall and other cellular components of Actinobacteria. Some common chemotaxonomic features that are used for identification are:

• Cell wall type: The cell wall type is determined by the presence or absence of certain amino acids and sugars in the peptidoglycan layer of the cell wall. Most Actinobacteria have cell wall type I, which contains meso-diaminopimelic acid (DAP) as the diagnostic amino acid and arabinose and galactose as the diagnostic sugars. However, some genera have different cell wall types, such as II (containing ornithine instead of DAP), III (containing lysine instead of DAP), IV (containing no DAP), V (containing no peptidoglycan), VI (containing mycolic acids), VII (containing pseudo murein), VIII (containing teichuronic acids), IX (containing teichoic acids), X (containing glycine), XI (containing alanine), XII (containing glutamic acid), XIII (containing muramic acid), XIV (containing aminobutyric acid), XV (containing diamino propanoic acid), XVI (containing diamino hexanoic acid), XVII (containing diamino octanoic acid), XVIII (containing diamino decanoic acid), XIX (containing diamino dodecanoic acid), XX (containing diamino tetra decanoic acid).

• G + C content: The G + C content is the percentage of guanine and cytosine bases in the DNA of Actinobacteria. Most Actinobacteria have a high G + C content ranging from 55% to 75%. However, some genera have lower G + C content, such as Bifidobacterium (40% to 60%), Eggerthella (46% to 54%), Collinsella (47% to 53%), etc.

• Isoprenoid quinones: Isoprenoid quinones are lipid-soluble compounds that act as electron carriers in cellular respiration. Most Actinobacteria produce menaquinones as their major quinones, which have different numbers of isoprene units attached to their naphthoquinone ring. The number and type of isoprene units can vary among different genera and species of Actinobacteria and can be used as chemotaxonomic markers.

• Fatty acids: Fatty acids are long-chain carboxylic acids that form part of the cell membrane lipids of Actinobacteria. The length and degree of saturation of fatty acids can vary among different genera and species of Actinobacteria and can be used as chemotaxonomic markers.

• Mycolic acids: Mycolic acids are long-chain fatty acids that contain one or more cyclopropane rings and form part of the cell wall lipids of some Actinobacteria belonging to the suborder Corynebacterineae (such as Corynebacterium, Mycobacterium, Nocardia, etc.). The structure and composition of mycolic acids can vary among different genera and species of Corynebacterineae and can be used as chemotaxonomic markers.

Genetic Methods

Genetic methods rely on the comparison of the nucleotide sequences or hybridization patterns of specific genes or regions of DNA among different strains or species of Actinobacteria. Some common genetic methods that are used for identification are:

• 16S rRNA gene sequencing: The 16S rRNA gene is a part of the small subunit ribosomal RNA that is involved in protein synthesis in bacteria. It is highly conserved among bacteria but also contains variable regions that reflect their evolutionary relationships. The 16S rRNA gene sequence can be obtained by PCR amplification using universal primers and then sequenced using Sanger sequencing or next-generation sequencing methods. The sequence can then be compared with reference databases such as GenBank or Ribosomal Database Project to identify the closest relatives among known Actinobacteria.

• Multilocus sequence analysis (MLSA): MLSA is a method that compares the sequences of several housekeeping genes that encode essential proteins involved in various cellular functions such as DNA replication, transcription, translation, etc. These genes are usually conserved within species but vary among different species or genera of bacteria. MLSA can provide higher

Actinobacteria are a diverse group of bacteria that have remarkable metabolic versatility and ecological importance. They play various roles in nature and human applications, as well as cause some harmful effects on plants and humans. Some of the significance of Actinobacteria are:

Roles in Nature

• They are significant decomposers in nature. They degrade complex organic substances like cellulose, lignin, keratin, chitin, hemicellulose, and other polymers, which help in soil fertility and vegetation.

• They play a major role in nutrient cycling in nature. They are involved in the decomposition of organic matter and the assimilation of inorganic matter, which affects the biogeochemical cycles of carbon, nitrogen, sulfur, and phosphorus.

• They are found in extreme environments and maintain the biogeochemical cycle in such harsh niches. They help sustain life forms in such conditions.

• They release several enzymes, vitamins, proteins, antimicrobials, and micronutrients that influence the soil and aquatic habitat microbial flora.

Human Applications

• Actinobacteria are the major source of antimicrobials. More than two-thirds of the clinically used antibiotics are derived from Actinobacteria, especially from Streptomyces spp. and Micromonospora spp. They also produce antifungal, antiparasitic, and anticancer compounds with medical applications.

• Actinobacteria are widely used in bioremediation of soil and water polluted with recalcitrant organic matter and hydrocarbons. They have the ability to degrade or transform various pollutants such as pesticides, herbicides, polycyclic aromatic hydrocarbons, polychlorinated biphenyls, explosives, etc.

• Actinobacteria are involved in composting of organic wastes. They act on the complex organic matter during the thermophilic phase of composting and produce humic substances that improve the quality of compost.

• Actinobacteria produce various enzymes and vitamins with industrial applications. They produce enzymes such as chitinases, catalases, L-asparaginase, proteases, ureases, keratinases, lipase, amylase, cellulases, xylanases, and phytases that are used in the fermentation, food, textile, leather, paper, and pulp industries. They also produce vitamin B12, which is used as a dietary supplement.

• Actinobacteria produce various biosurfactants that have to emulsify, foaming, wetting, and dispersing properties. These biosurfactants are used for the production of detergents, herbicides, cosmetics, antimicrobials, etc., and are also used in the paper, pulp, textile, and food industries.

• Actinobacteria produce various bioherbicides, bioinsecticides, and larvicides that are used to control unwanted weeds, insects, and larvae. These compounds are environmentally friendly alternatives to synthetic pesticides.

• Actinobacteria produce various plant growth hormones that promote plant growth and development. They also produce antimicrobials that protect plants from pathogens. Some Actinobacteria, such as Frankia spp., can fix atmospheric nitrogen and form symbiotic associations with plants.

• Actinobacteria produce various nanoparticles that have pharmaceutical properties. They can synthesize silver, gold, zinc, copper, and magnesium nanoparticles that have antibacterial, antifungal, anticancer, antioxidant, and anti-inflammatory activities.

• Actinobacteria produce various pigments that have colorant properties. They can produce pigments such as rhodopsin,

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Harmful Effects of Actinobacteria on Plants and Humans

Although actinobacteria are beneficial for many aspects of agriculture and biotechnology, they can also cause harm to plants and humans in some cases. Actinobacteria are responsible for various plant diseases, such as scab, common scab, potato scab, soft rot, crown gall, fire blight, and leaf spot. Some of the common actinobacterial pathogens and the diseases they cause are listed in Table 1.

| Actinobacterial Pathogen | Plant Disease | Host Plant |

| --- | --- | --- |

| Streptomyces scabies | Scab | Potato, carrot, beet |

| Streptomyces acidiscabies | Common scab | Potato |

| Streptomyces turgidiscabies | Potato scab | Potato |

| Streptomyces ipomoea | Soft rot | Sweet potato |

| Agrobacterium tumefaciens | Crown gall | Many dicotyledonous plants |

| Agrobacterium rhizogenes | Hairy root disease | Many dicotyledonous plants |

| Rhodococcus fascians | Leafy gall disease | Many herbaceous plants |

| Rhodococcus erythropolis | Fire blight | Pear, apple |

| Curtobacterium flaccumfaciens pv. flaccumfaciens | Bacterial wilt and canker | Bean |

| Clavibacter michiganensis subsp. michiganensis | Bacterial canker and wilt | Tomato |

| Clavibacter michiganensis subsp. sepedonicus | Ring rot | Potato |

| Clavibacter michiganensis subsp. insidious | Bacterial wilt and stem rot | Alfalfa |

| Clavibacter michiganensis subsp. nebraskensis | Goss`s wilt and leaf blight | Corn |

Table 1: Some common actinobacterial pathogens and the plant diseases they cause.

Actinobacteria can also infect humans and cause serious diseases, especially in immunocompromised individuals. Some of the most important human pathogens belong to the genera Mycobacterium, Nocardia, Corynebacterium, Tropheryma, and Propionibacterium. Some of the common human diseases caused by actinobacteria are listed in Table 2.

| Actinobacterial Pathogen | Human Disease |

| --- | --- |

| Mycobacterium tuberculosis | Tuberculosis |

| Mycobacterium leprae | Leprosy |

| Mycobacterium avium complex (MAC) | Pulmonary disease, disseminated infection |

| Mycobacterium ulcerans | Buruli ulcer |

| Nocardia asteroides complex (NAC) | Nocardiosis (pulmonary, cutaneous, disseminated) |

| Nocardia brasiliensis | Actinomycetoma (chronic granulomatous infection of skin and subcutaneous tissue) |

| Corynebacterium diphtheriae | Diphtheria (respiratory tract infection with pseudomembrane formation) |

| Corynebacterium jeikeium | Septicemia, endocarditis, wound infection |

| Corynebacterium urealyticum | Urinary tract infection, urolithiasis |

| Tropheryma whipplei | Whipple`s disease (systemic infection with malabsorption syndrome) |

| Propionibacterium acnes | Acne vulgaris (inflammation of sebaceous glands), endophthalmitis, endocarditis |

Table 2: Some common actinobacterial pathogens and the human diseases they cause.

Therefore, actinobacteria have both positive and negative impacts on plants and humans. It is important to identify and control the harmful actinobacteria while exploiting the beneficial ones for various purposes.

We are Compiling this Section. Thanks for your understanding.