Actinobacteria is a phylum of Gram-positive bacteria characterized by high G+C DNA content. As one of the largest bacterial phyla, the group consists of diverse organisms that can be found in aquatic and terrestrial ecosystems. They also exhibit high metabolic versatility which has had a positive impact on the environment.
Some species within the phylum Actinobacteria include:
· Iamia majanohamensis
· Olsenella uli
· Cryptobacterium curtum
· Thermoleophilum album
· Gaiella occulta
· Rubrobacter aplysinae
* The majority of species in the phylum Actinobacteria are Gram-positive. However, there are a few Gram-negative species in the group (e.g. Saccharomonospora viridis, Ferrithrix thermotolerans, and Ferrimicrobium acidiphilum).
The phylum Actinobacteria is a large group that consists of diverse organisms.
They are divided into several groups that include:
Thermophilic bacteria are species that can survive and grow in relatively high-temperature habitats (between 40 and 80 degrees C). Members of this group are also divided into two main categories namely, strictly thermophilic (those that prefer temperature ranges between 55 and 60 °C) and moderately thermophilic species which prefer temperature range between 45 and 55 °C.
Thermophilic Actinobacteria can be found in:
· Moldy hay
· Decaying vegetables
· Compost heaps
Thermophilic bacteria are also strictly aerobes. As such, they need oxygen for growth. As mentioned, the majority of thermophilic Actinobacteria species can be found in moldy hay, sugar cane bagasse, and decaying vegetable materials, etc. This is because they are obligate chemoorganotrophs and depend on decaying organic matter as their source of energy.
However, a number of studies have identified several obligate chemoautotrophs (e.g. Streptomyces thermoautotrophicus and Acidithiomicrobium species). These species can use carbon dioxide, hydrogen, and sulfur as a source of energy.
Some of the other species are classified as facultative chemoautotrophs (e.g. Streptomyces) and facultative methylotrophy (e.g. Amycolapsis methanolica). Whereas facultative chemoautotrophs can also use organic matter for nutrition, facultative methylotrophs can use single carbon compounds (and other organic matter) as a source of energy.
Generally, thermophilic bacteria have been found in a number of habitats across the world such as the Mongolian Desert (Steppe Zone) and the subtropical region of Argentina.
However, this is largely influenced by availability of nutritional sources that given species require for growth and proliferation. For instance, Acidithiomicrobium species can be found in hydrothermal vents where they can access sulfur.
A good number of thermophilic Actinobacteria also fast-growing and produce spores.
They are divided into several groups that include:
· Monosporic thermophilic actinobacteria - Members of the genera Micromonopsora, Saccharomonopora, and Thermomonospora
· Bisporic thermophilic Actinobacteria - Members of the genera Microbispora and Thermobispora
· Oligosporic thermophilic Actinobacteria - Members of the genera Streptomyces and Thermopolyspora
* Spores produced by these bacteria can tolerate high-temperature ranges for a few days.
Thermophilic Actinobacteria have several characteristics that allow them to survive and thrive in their respective habitats (habitats with relatively high-temperature range). Along with high GC content in their genome, the majority of these bacteria have a higher quantity of charged amino acids like Glutamic acid, Arginine, Lysine, and Leucine. These are suspected to promote thermotolerance among thermophilic bacteria.
In some of the species (e.g. Saccharomonospora xinjiangensis), researchers have identified a unique phospholipid in their cell wall that promotes growth at relatively high temperature.
* With the exception of a few species, the majority of thermophilic bacteria are non-motile, non acid-fast, aerobes.
Commonly found in terrestrial environments (e.g. acidic forests with a pH range value of between 3.5 and 6.5), acidophilic Actinobacteria are species that generally prefer acidic conditions. They are divided into two main groups that include; neutrotolerant acidophiles (prefer pH range value of between 5.0 and 5.5) and strict acidophiles which prefer a pH value of 4.5. They can grow in areas with pH value of between 3.5 and 6.5.
Like other Actinobacteria, acidophilic species are also characterized by high GC content. Some of the species classified as acidophilic Actinobacteria belong to the genera Streptomyces, Micromonospora, Acidothermus, and Ferrimicrobium among a few others.
As filamentous bacteria, they form long strands that consist of single cells growing end to end. In culture, they have been shown to grow slowly and often produce pigments.
Some of the other characteristics observed under the microscope include:
· Calatase positive
· Can reduce and degrade a number of materials including nitrates, esculin, gelatin, and starch, etc.
· Contain large amounts of L-diaminopimelic acid in their cell wall
· Have complex polar lipid patterns
As the name suggests, alkaliphilic Actinobacteria are species that commonly reside in alkalic environments. For instance, they can be found in alkaline soil and soda lakes. However, some of the species are found in neutral environments.
Generally, members of this group are divided into three main groups that include; alkalipihic species - species that thrive in areas with a pH range of between 10 and 11, moderately alkaliphilic Actinobacteria - those found in areas with pH range of between 7 and 10, and alkalitolerant Actinobacteria - found in environments with a pH range of between 6 and 11.
The majority of species found on land (terrestrial environment) are aerobic. Others can be found in aquatic habitats (e.g. Microbacterium sediminis found in deep-sea) where they exist as microaerobes or as facultative anaerobes. Like many other Actinobacteria, alkaliphilic and alkalitolerant species are non-motile.
While a good number can form spores, studies have identified several species that do not.
Halophilic Actinobacteria are microorganisms that can grow in environments with saturated salt. Currently, members of this group are divided into four main groups.
* Some of the environments relatively high salt concentrations include seawater, salty lakes, and saline soils, etc.
* Some examples of halophilic Actinobacteria include some members of the genera Streptomyces, Micromonospora, and Rhodococcus.
Though halophilic Actinobacteria can grow in environments with different levels of salt concentration, they also have to protect themselves from extremely high salt conditions (hyper-salt conditions).
In the majority of species, researchers have identified adaptive systems on their cell membranes that prevent sodium chloride from entering the cells. For instance, in a salty aquatic environment, these systems prevent inorganic salts and water-soluble low molecular weight organic compounds from penetrating into the cell through the membrane.
* Like many other Actinobacteria, the majority of Halophilic Actinobacteria have high G+C content in their genome. They are also Gram-positive bacteria with the majority of species being non-motile.
Endophytic Actinobacteria include species found living in different types of plants. Here, some of the most common species belonging to the genera Streptomyces, Nocardia, and Microbispora, etc.
Within the plants, these bacteria benefit from favorable conditions that promote growth. In turn, they have been shown to protect the plant against various diseases and plants especially at the roots.
The phylum Actinobacteria also consists of many species involved in nitrogen fixation. Most of these species belong to the genus Frankia. Some of the plants that form a symbiotic relationship with these species include bitterbrush, bayberry, and cliffrose, etc.
* Some Actinobacteria are classified as endosymbiotic bacteria (e.g. some members of the genera Kocuria, Dietzia, and Gordonia etc.). These species can be found in the cells of the plant host (or in certain parts of the plant's body). Some of the plants or bacteria rely on this relationship for their survival.
Some Actinobacteria can be found in the gut of vertebrates and invertebrate animals. These bacteria tend to form a symbiotic relationship with the host where they benefit from conducive conditions for growth (nutrients, favorable temperature range, etc.) and protect the host from pathogenic bacteria as well as their role in detoxification, etc. in return.
Actinobacteria is a highly diverse group with species that can be found in different environments across the world. They also exhibit significant metabolic diversity depending on where they are found. However, the majority of species share a number of similar characteristics.
Gram-positive - Though some species have unique components in their cell wall that allow them to survive in their environment, the majority of Actinobacteria are Gram-positive in nature. As such, their cell wall consists of a thick peptidoglycan layer. However, some species have been described as Gram-variable because they are neither Gram-positive nor gram-negative.
High Guanine and Cytosine content - As mentioned, the majority of Actinobacteria species are characterized by high G+C content in their genome (about 60 percent in some species). High G+C content in these bacteria has been associated with their diversity and ability to colonize different environments/ecologies across the world.
Filamentous morphology - A good number of Actinobacteria are filamentous and can form branching filaments (resembling mycelia). These filaments vary in shape and color which makes it possible to distinguish some of the species.
For instance, some of the species form filaments with open loops which others have closed loops - The filaments/mycelium might be substrate which shows variation in size, shape, and thickness - They also vary in color from orange and pink to yellow and green etc., or aerial (thicker than substrate mycelium),
* The majority of Actinobacteria are mesophilic. The species grow in areas with a temperature range between 25 and 30°C.
* The majority of Actinobacteria produce substrate mycelium. However, they can form aerial hyphae when cultured on solid surfaces. Reproductive spores are produced on these hyphae (aerial hyphae).
The cells also exhibit different morphologies, ranging from coccoid/spherical (e.g. Micrococcus species) to rod-coccid (E.g. Members of the genus Arthrobacter).
Because of their diversity in nature, Actinobacteria species produce a wide variety of products that have found many uses.
Today, members of the phylum Actinobacteria are estimated to produce about 80 percent of all antibiotics (about 10,000 compounds). The majority of these products are produced by species in the genus Streptomyces and Micromonospora. Some examples of these antibiotics include actinomycin, streptomycin, and streptothricin.
Normally, these products are produced as secondary metabolites by these bacteria.
Generally, antibiotics from these bacteria are divided into six main categories that include:
Many Actinobacteria species produce a variety of enzymes used for degrading organic material. Today, some of these enzymes are used in many industries (textile and paper industries and the food industry).
Some of the enzymes used in various industries including:
Amylase - Produced by some Thermomonospora species and is used in the textile, baking, detergent, and paper industries.
Protease - Produced by some Saccharomonospora and Nocardiopsis species, the enzyme protease is commonly used in the pharmaceutical, leather, brewing, and detergents industry.
Keratinase - Produced by some Actinomadura and Thermomonospora species. The enzyme is used in the leather and Pharmaceutical industries.
Xylanase - This enzyme is produced by some Streptomyces and Thermomonospora. It's used in the paper and baking industries.
Some of the other enzymes produced by these bacteria include:
* Many of the hydrolytic enzymes produced by Actinobacteria can degrade such biopolymers as xylan, chitin, and cellulose, etc. As such, they play an important in the global carbon cycle.
On the other hand, they have also been used to produce biofuels through the decomposition of plant biomass to simple sugars using the cellulolytic enzymes they produce.
Herbicides are chemicals used to destroy unwanted weeds. Some species in the phylum Actinobacteria can produce a number of products that can act against these plants.
Herbimycins and herbicidines - These are produced by Streptomyces saganonensis and can be used to control monocotyledonous and dicotyledonous weeds.
Anisomycin - Produced by some Streptomyces species, this compound is used to control a variety of plants including broad-leaved weed, grassy weeds, and crabgrass, etc.
Biosurfactants are surface-active molecules that serve to reduce surface/interfacial tension. They are used in many areas including mining, oil recovery, textile, and pharmaceutical, etc.
Compared to synthetic surfactants, biosurfactants have low toxicity, remain active at extreme salinity and pH values, and tend to be highly degradable. For these reasons, they are better for the environment compared to synthetic ones.
As mentioned, some Actinobacteria produce pigments that have been used for their identification. By growing these bacteria in culture, researchers have been able to obtain natural dyes of varying colors (red, rose, green, and brown, etc.).
Compared to synthetic dyes, these dyes are less hazardous. They can be used in various industries including textile.
* In recent years, researchers have been trying to find ways through which products from these bacteria can be used to control plant diseases, control nematodes, and promote plant growth among many other applications.
Bacteriology as a field of study
Doris A. van Bergeijk et al. (2020). Ecology and genomics of Actinobacteria: new concepts for natural product discovery.
Essaid Ait Barka et al. (2016). Taxonomy, Physiology, and Natural Products of Actinobacteria.
L. Shivlata and Tulasi Satyanarayana. (2015). Thermophilic and alkaliphilic Actinobacteria: biology and potential applications.
Stackebrandt E., Schumann P. (2006) Introduction to the Taxonomy of Actinobacteria.