Actinobacteria: What are they? Types, Terrestrial Environment, Aquatic Environment and General Characteristics

They are a species of Gram-positive bacteria.

The ancient Actinobacteria species is composed of phylogenetically and physiologically diverse bacteria that help the earth’s ecosystems function.

As free-living organisms and symbionts of herbivorous animals, actinobacteria contribute to the global carbon cycle by decomposing plant biomass.

In addition, they mediate the dynamics of the community as producers of small molecules with diverse biological activities.

The evolution of high cellulolytic capacity and diverse chemistry, shaped by their ecological roles in nature, makes the Actinobacteria a promising group for the bioenergy industry.

Specifically, its enzymes can contribute to the decomposition on an industrial scale of the biomass of cellulosic plants into simple sugars that can then be converted into biofuels.

In addition, its ability to biosynthesize a range of small molecules has the potential for the production of specialized biofuels.


Actinobacteria are ubiquitous and one of the most diverse groups of bacteria in nature. Its members range from single-cell anaerobic organisms to aerobic, filamentous, and spore-forming lineages. Alone, the genus Streptomyces represents almost 5% of the ~ 16,000 bacterial species described.

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 the soil, they behave similarly to fungi, which helps break down the organic matter of dead organisms so that plants can absorb the molecules again.

The colonies often grow extensive mycelia in this role, as would a mushroom. The name of a significant order of the species, Actinomycetales (the actinomycetes), reflects that it was believed to be fungi for a long time.

Some soil actinobacteria (such as Frankia) live symbiotically with plants whose roots permeate the soil, fixing nitrogen for plants in exchange for access to some of the saccharides in the plant.

Beyond the great interest in Actinobacteria for its role in the soil, there is still much to learn about them. Although currently mainly understood as soil bacteria, they could be more abundant in freshwater.

Actinobacteria is one of the dominant bacterial species and contains one of the largest bacterial genera, Streptomyces. Streptomyces and other Actinobacteria are the main contributors to the physical storage of soils. They are also the source of many antibiotics.

Although some of the largest and most complex bacterial cells belong to Actinobacteria, it has been described that the Marine Actinomarinal group possesses the smallest free-living prokaryotic cells.

It represents gram-positive bacteria with a high content of guanine plus cytosine (G + C) in their DNA. This bacterial group includes microorganisms that exhibit a broad spectrum of morphologies, from coccoid to fragmented hyphae forms, and have variable physiological and metabolic properties.

In addition, members of Actinobacteria have adopted different lifestyles, and maybe pathogens (e.g., Corynebacterium, Mycobacterium, Nocardia, Tropheryma, and Propionibacterium), soil dwellers (Streptomyces), plant eaters (Leifsonia) or gastrointestinal commensals (Bifidobacterium) ).

The divergence of Actinobacteria from other bacteria is old, making it impossible to identify the bacterial group phylogenetically closest to Actinobacteria.

Sequence analysis of the genome has revolutionized all aspects of bacterial biology by improving understanding of bacteria’s genetics, physiology, and evolutionary development.

Several genomes of Actinobacteria have been sequenced, revealing a wide genomic heterogeneity, probably reflecting their biodiversity.

Actinobacteria have substantially positively contributed to human health; they produce many compounds used as essential medicines, including most antibiotics.

In addition, the species includes essential pathogens, such as Mycobacterium tuberculosis, the causative agent of one of the most devastating diseases in the history of humanity, tuberculosis.

Due to these biomedical impacts, Actinobacteria have avoided mainly the darkness relegated to most bacterial species, receiving a critical scientific study of their genetics, molecular biology, and biochemistry.

In addition to their significant influence on human health, Actinobacteria have vital ecological functions.

Before focusing on the discovery of antibiotics, the work of Nobel Prize winner Selman Waksman on soil bacteria and its impact on agricultural productivity was one of the first to implicate Actinobacteria as essential contributors in the process of decomposition of plant biomass.

More recently, Actinobacteria have been revealed as widespread symbionts of eukaryotes, which help herbivores gain access to plant biomass as nutritional mutualists and produce natural products as defensive mutualists.

The studies of Actinobacteria as defensive mutualists have led to the discovery of new antibiotics with possible pharmaceutical applications, renewing the recognition of the value of understanding the ecology of Actinobacteria for the discovery of drugs.

Actinobacteria are implicated as important decomposers of plant material in nature. Their cellulolytic enzymes can be used to more efficiently decompose plant biomass into simple sugars, which can then be used to produce biofuels.

In addition, the diverse biosynthetic capacity of Actinobacteria, which evolved to mediate its environmental interactions, could be exploited to produce a range of bioproducts, including unique biofuel compounds.

Most Actinobacteria of medical or economic importance are found in the subclass Actinobacteridae and belong to Actinomycetales. While many of these cause diseases in humans, Streptomyces is notable as a source of antibiotics.

Gardnerella is one of the most researched of those Actinobacteria not in the Actinomycetales. The classification of Gardnerella is controversial, and MeSH catalogs it as a Gram-positive and Gram-negative organism.

Actinobacteria, especially Streptomyces spp, are recognized as producers of many bioactive metabolites beneficial for humans in medicine, such as antibacterial, antifungal, antiviral, antithrombotic, immunomodifier, antitumor, and enzyme inhibitors.

And in agriculture, including insecticides, herbicides, fungicides, and growth-promoting substances for plants and animals.

The antibiotics derived from Actinobacteria that are important in medicine include aminoglycosides, anthracyclines, chloramphenicol, macrolides, tetracyclines, etc.

Actinobacteria have a high content of guanine and cytosine in their DNA. The guanine content plus cytosine (G + C) of Actinobacteria can reach 70%, although some may have a low guanine content plus cytosine (G + C).

The analysis of the sequence of glutamine synthetase has been suggested for the phylogenetic analysis of Actinobacteria.

Terrestrial environment

The soil remains the most critical habitat for Actinobacteria with streptomycetes that exist as a significant component of its population. According to numerous reports, Streptomyces was the most abundant genus isolated in each study.

The Terrestrial Actinobacteria have several attractive antimicrobial potentials. Oskay et al. isolated Actinobacteria that could produce new antibiotics with high antibacterial activity.

In the anoxic mangrove rhizosphere, Actinobacteria species such as Streptomyces, Micromonospora, and Nocardioform was found 1000 to 10000 times smaller than the arable land to the influence of the tides.

Similarly, Nocardia isolated from the mangrove soil produced new cytotoxic metabolites that strongly inhibited human cell lines, such as gastric adenocarcinoma.

Dessert soil is also considered an extreme terrestrial environment where only certain species, mainly Actinobacteria, often use Microcoleus as a food source.

Several reports are showing the distribution of Actinobacteria in several places, such as sandy soil (Cario, Egypt), black alkaline soil (India), sandy loam soil (Nigeria), alkaline dessert soil (Egypt), and subtropical desert soil (Thar, Rajasthan), where Streptomyces sp. they were dominant followed by other organisms, such as Nocardia, Nocardiopsis and Actinomycetes.

In the study by Nithya et al., 134 morphologically different culturable Actinobacteria were isolated from 10 different desert soil samples. It was found that the isolates had a variable level of antibacterial activity against bacterial pathogens.

Likewise, actinobacteria play an essential role in the microbial community of the rhizosphere in the renewal of recalcitrant plant organic matter. Therefore, the rhizosphere region is considered one of the best habitats for the isolation of these microorganisms.

Priyadarshini et al. in their study, isolated 45 morphologically distinct colonies from 12 different paddy soils and observed their ability to inhibit the growth of Cyperus rotundus.

Los aislados incluyen Streptomyces sp., Streptoverticillium sp., Actinomadura sp., Kitasatosporia sp., Nocardiopsis sp., Pseudonocardia sp., y Kibdelosporangium sp.

Aquatic environment

Actinobacteria are widely distributed in aquatic habitats, which can sometimes be washed from the surrounding terrestrial habitats.

It is vitally important that the numbers and types of Actinobacteria be interpreted in light of information on organisms, such as Thermoactinomyces and Rhodococcus coprophilous, which are known to be good indicators of the terrestrial component of actinobacteria propagules in water and sediments.

Thermoactinomyces resistant endospores are produced in self-heating compost, superheated fodder, and surface soil but can be washed in aquatic habitats where they are deposited in sludge and sediments.

It has been assumed that these thermophiles can not grow at ambient temperatures in most aquatic habitats. Similarly, the resting coccus stage of R. coprophilous moves into marine and freshwater habitats, where it can survive but probably does not grow.

The environment of actinobacteria in freshwater:

Cross, in his study, showed that Actinobacteria could be easily isolated from freshwater sites. Some of the main types of Actinobacteria that inhabit freshwater include Actinoplanes, Micromonospora, Rhodococcus, Streptomyces, and Thermoactinomyces forming endospores.

Actinoplanes are commonly found in soils, rivers, and lakes, and the spore vesicles of these organisms can withstand prolonged desiccation, but they release their mobile spores for dispersal when rehydrated.

Zoospores are mobile, using a tuft of flagella that present chemotaxis and require an exogenous energy source. It is also considered that micromonospores are a common freshwater Actinobacteria. They are indigenous to these habitats, in which they produce cellulose, chitin, and lignin.

Numerous reports confirmed the presence of Micromonospora in streams, rivers, and fluvial sediments and considered them an integral part of the aquatic microflora.

Johnston & Cross discovered that streptomycetes did not grow in several lakes, especially in deeper mud layers where predominant micromonospores.

While another study by AI-Diwany and Cols showed a significant correlation between micromonospores and thermoactinomycetes isolated from the Wharfe River in West Yorkshire, where a more substantial number of micromonospores were found in the adjacent soil.

One study revealed that Micromonospora spores were washed in freshwater habitats where they can remain dormant for several years.

Aquatic streptomycetes have been claimed, but AI-Diwany and Colsfound a high degree of correlation between the counts of streptomycetes, fecal streptococci, and Rhodococcus bacteria.

Other freshwater inhabitants include Actinomadura Madurai, Mycobacterium kansasii, Arthrobacter, Corynebacterium, and Nocardia species.

The concentration of hydrophobic spores and hyphae at the water/air interface can increase the number of streptomycetes, micromonospores, and Rhodococci in foam in the river water.

The evidence clearly shows that Actinobacteria can be activated in freshwater ecosystems in the presence of substrates and conditions suitable for growth instead of specifically adapting to live in such an environment.

The environment of Actinobacteria in seawater:

When comparing the diversity of Actinobacteria in the terrestrial environment, the most incredible biodiversity is found in the oceans. The marine environment is an unexploited source of new Actinobacteria diversity and, therefore, new metabolites.

Marine actinobacteria that live in significantly different environments produce different types of bioactive compounds than terrestrial ones.

Marine Actinobacteria had to adapt to extremely high pressure and anaerobic conditions at temperatures below 0-8 ° C in the deep seabed at high acidic temperatures above 8-100 ° C near hydrothermal vents in the ocean ridges.

Actinobacteria represent approximately 10% of the bacteria that colonize marine organic aggregates. Their antagonistic activity could be highly significant in maintaining their presence, which affects the degradation and mineralization of organic matter.

Several recent investigations have confirmed the presence of marine actinobacteria native to the oceans and the distribution of marine actinobacteria in different environments and habitats.

General characteristics of Actinobacteria

Actinobacteria comprises a group of branched unicellular microorganisms, most aerobic forming mycelia known as substrate and antenna.

They reproduce by binary fission or by the production of spores or conidia, and the sporulation of Actinobacteria is through fragmentation and segmentation or the formation of conidia.

The morphological appearance of Actinobacteria is compact, often leathery, giving a conical appearance with a dry surface in the culture media, and usually covered with aerial mycelium.

Aerial mycelium:

The aerial mycelium is generally thicker than the substrate mycelium. The aerial mycelium shows a sufficient differentiation so that a varied assortment of isolates can be segregated into several groups with similar morphological characteristics under fixed conditions.

This is designated as one of the essential criteria for classifying the genus Streptomyces in species, which comprises a structure (cottony, velvety, or powder), ring formation, concentric areas, and pigmentation.

Substrate mycelium:

The substrate mycelium of Actinobacteria varies in size, shape, and thickness. Its color ranges from white or practically colorless to yellow, brown, red, pink, orange, green, or black.

Morphological appearance:

Morphology has been an essential feature in identifying Actinobacteria isolates, used in the first descriptions of Streptomyces species. This is done using several traditional cultural media.

Types of Actinobacteria

Actinobacteria thermophilic:

Researchers have carried out numerous studies to confirm the existence of extremophile actinobacteria and highly tolerant to the soil (susceptible to acids and accessible to alkalis, psychrotolerant and thermotolerant, halotolerant haloalcalorie or xerophilous).

Mesophilic Actinobacteria can grow at an optimum temperature of 20 ° С to 42 ° С, among which there are thermotolerant species, which can survive at 50 ° С.

Moderately thermophilic Actinobacteria have optimal growth at 45 ° C-55 ° C, whereas strictly thermophilic Actinobacteria grow at 37 ° C-65 ° C with the optimum temperature at 55 ° C-60 ° C.

The incubation temperatures of 28 ° С, 37 ° С, and 45 ° C are considered optimal for the isolation of mesophilic, thermotolerant, and moderately thermophilic actinobacteria in the soil.

Thermoactinomies, which are currently excluded from the order Actinomycetales, are described as thermophilic forms based on their phenotypic and molecular genetic characteristics and between some species of Thermomonospora, Microbispora, Saccharopolyspora, Saccharomonospora, and Streptomyces.

Actinobacteria acidófila:

Acidophilic actinobacteria, common in terrestrial habitats, such as acid forest and mine drainage soil, grow in about 3.5 to 6.5, with optimal rates at pH 4.5 to 5.5.

Acidophilic Actinobacteria have been shown to consistently form two distinct aggregate taxa (neutral acidophilic and strictly acidophilic cluster groups) based on numerical phenetic data; the members of the two groups share common morphological characteristics chemotaxonomic properties.

Also, some members of the strictly acidophilic group form a distinct taxon, such as the genus Streptacidiphilus, which has been assigned to the revised family Streptomycetaceae, together with the genera Kitasatospora and Streptomyces.

Actinobacteria halófila:

Halophilic actinobacteria are classified according to their growth in media containing different salt concentrations.

Extreme halophiles grow best in media containing 2.5-5.2 M salt, while extreme halophiles in the limit grow better in media containing 1.5-4.0 M salt, moderate halophiles grow better in media containing salt 0.5-2.5 M, and finally halotolerant that do not show an absolute content of salt requirement for growth, but grow well to very high salt concentrations and tolerate 100 g / l of salt.

Seawater, saline soils, salt lakes, brines, and alkaline saline habitats are considered the best to isolate Actinobacteria halófila.

In general, most halophilic actinobacteria have been isolated from saline soils. Halophilic actinobacteria isolated from marine environments are assigned to some genera, including Micromonospora, Rhodococcus, and Streptomyces.

Actinobacteria endopítica:

The endophytic actinobacteria inhabit the inner part of the plants, apparently causing changes not visible in their hosts. These actinobacteria play specific roles, for example, protecting host plants against insects and diseases.

The endophytic actinobacteria constitute a large part of the rhizosphere, also found in plants. The species studied extensively are of the genus Frankia, nitrogen-fixing bacteria of non-leukocytic plants, and some species of the genus Streptomyces that are phytopathogens.

Generally, endophytic actinobacteria include Streptomyces, but the genera Nocardia, Micromonospora, Kitasatospora, Pseudonocardia, Microbispora, Nocardioides, Actinomadura, Promicromonospora, and Streptosporangium are also found in plants, such as Palicourea longifolia, Calycophyllum acreanum, Monstera spruceana, Croton lechleri, Cantua buxifolia, Siparuna crassifolia, and Eucharis cyaneosperma.

Symbiotic actinobacteria:

About 15% of the world’s nitrogen is naturally set by the symbiotic relationships between several Frankia species that belong to the Actinobacteria family.

Plants that form symbiotic relationships with Frankia are called actinorhizal plants. Researchers have found more than 160 plants with Actinobacteria as their host, including alders, Russian olive, sweet fern, bitterbrush, and rose cliffs.

Frankia can provide most or all of the nitrogen needs of the host plant. Numerous Frankia species, including Casuarina, isolates form nitrogen-reducing vesicles in vitro and plant.

These nitrogen-fixing bacteria and their host plants are often pioneer species in young soils with nitrogen deficiency and altered, such as moraines, volcanic flows, and dunes.

Actinobacteria endosímbionica:

An endosymbiont is any organism that lives inside the body or cells of another organism. The process of endosymbiosis is sometimes mandatory; that is, the endosymbiont or the host can not survive without the other.

Members of the Actinobacteria species have been identified as abundant members of microbial communities associated with sponges.

However, Actinobacterium endosymbionts have also been reported in other animals, such as Hylobates hoolock, Panthera tigris altaica, Ailurus fulgens, Cavnlvara Ursidae, Ursus thibetanus, Cervus elaphus, Elaphurus davidianus, and Vicugna pacos.

Gut actinobacteria:

Although Actinobacteria are found in diverse habitats, some are also known to form intimate associations with invertebrates and vertebrates.

Symbiotic interactions are essential for survival and reproduction because they play a crucial role in nutrition, detoxification of certain compounds, growth performance, and protection against pathogenic bacteria.

Many studies have shown that some symbiotic species of actinobacteria, probiotics, control bacterial diseases in livestock, poultry, and aquaculture.

They also take part in the host’s health by converting the food into microbial biomass and final fermentation products that the animal host can use.

Applications of Actinobacteria

Actinobacteria are well recognized for their production of primary and secondary metabolites that have critical applications in various fields. They are also a promising source of a wide range of essential enzymes produced on an industrial scale.

A large part of the antibiotics in the market is obtained from Actinobacteria. They produce enzymatic inhibitors useful for cancer treatment and immune modifiers that improve the immune response.

They can degrade a wide range of hydrocarbons, pesticides, and aliphatic and aromatic compounds. They perform microbial transformations of organic compounds, a field of great commercial value.

Members of many genera of Actinobacteria can be potentially used to bioconvert underutilized agricultural and urban wastes into high-value chemical products.

Actinobacteria are also crucial in plant biotechnology since strains with antagonist activity against plant pathogens are useful in biological control. Its metabolic potential offers a vital area for research.


Actinobacteria play an essential role in producing a variety of medications that are extremely important for our health and nutrition.

Recently, diseases due to pathogenic bacteria resistant to multiple drugs are increasing robustly, so the search for new antibiotics is effective against pathogens resistant to various drugs.


Actinobacteria, both marine and terrestrial, produce many biologically active enzymes. They secrete amylases outside the cells, which helps them carry out extracellular digestion.

This enzyme is essential in biotechnological applications such as the food industry, fermentation, and textile and paper industries due to its capacity to degrade starch.

Another critical aspect of Actinobacteria is the production of cellulases, a collection of hydrolytic enzymes that hydrolyze the glycosidic bonds of cellulose and related cello-disaccharide derivatives.

The lipase is produced from several Actinobacteria, bacteria, and fungi and used in detergent, food, oleochemicals, environments, diagnostic and pharmaceutical fields in industries.

Many Actinobacteria have been isolated from several natural sources and in plant tissues and rhizospheric soil. The biological functions of Actinobacteria depend mainly on the sources from which the bacteria are isolated.

It is known that Actinobacteria, particularly streptomycetes, secrete multiple proteases in the culture medium.

Similarly, Actinobacteria are an excellent resource for L-asparaginase, produced by a range of Actinobacteria, mainly isolated from soils, such as S. griseus, Streptomyces karnatakensis, Streptomyces albidoflavus, and Nocardia sp.

The roots and rhizomes of various Thai medicinal plants, such as lemongrass and ginger, have long been used in traditional Thai medicine for stomach pain and asthma treatment.

The soil of the rhizosphere of these plants can be an attractive source of actinobacteria, which can produce new secondary metabolites. Enzymes such as catalase, chitinase, and urease are also made from Actinobacteria.

Actinobacteria isolated from the intestine of chickens and goats showed the presence of various enzymes such as amylase, protease, phytase, and lipase.


Despite several other essential applications, attention has been paid to marine Actinobacteria for its use as probiotics.

Probiotics are the live microbial complement that has a beneficial effect on the host through various means, such as modifying the environmental or host-associated microbial community, ensuring better use of the food, or improving its nutritional value.

It improves the host’s response to the disease or improves its ecological environment’s quality.

Recently, some studies were conducted on the possible use of marine Actinobacteria to prevent diseases against aquatic pathogens.

An antibiotic product extracted from Actinobacteria marina was incorporated into the diet to observe the effect in vivo on the white spot syndrome virus in the black tiger shrimp.

Adding peptide pheromones:

Aggregation is one of the essential criteria for selecting an excellent probiotic candidate, which is the process of reversible accumulation of cells with one or more strains.

For this aggregation process to occur, the production of pheromones is one of the main criteria that involves the defense against predators, the selection of pairs, and the overcoming of host resistance through massive attacks.

In particular, it has been shown that sex pheromone peptides in culture supernatants promote aggregation not only with the same species but also with related species.

Therefore, the ability of autoaggregation of a probiotic is a prerequisite for colonization of the gastrointestinal tract, while coaggregation provides close interaction with pathogenic bacteria.