Actinomycetes belong to the group of prokaryotic, gram-positive bacteria those are having a filamentous structure. Its filamentous structure resembles the fungal mycelium, which consists of a highly dense and filamentous network. Due to the filamentous structure of actinomycetes, it is also termed as thread or ray bacteria.
The cell wall and the internal structure of the actinomycetes are similar to the group of bacteria. Thus, actinomycetes refer to the filamentous actinobacteria that serve as a connecting link between the bacteria and fungi and shows resemblance with both.
They are the true bacteria (not fungus) and placed in the kingdom “Bacteria” and a class “Actinobacteria”. These are ubiquitous in nature (commonly found in soil). In addition to soil, they are also very common in marine habitat and considered as a treasure house of secondary metabolites.
Actinomycetes behave like decomposers, which carry out the degradation of organic compounds like chitin, complex sugars, hemicellulose etc. Its filamentous forms are predominantly aerobic, while a few are anaerobic. Here, you will get to know the definition, characteristics, life cycle, classification and economic importance of the actinomycetes.
Definition of Actinomycetes
Actinomycetes refer to prokaryotic or unicellular organisms that are having a gram-positive cell wall. The morphology of actinomycetes is more or less similar to the fungi because they can also produce a filamentous, dense, branched and raised colony over the substrate like fungi.
Most of its features are common to bacteria than that of fungi, due to which actinomycetes have been placed in the group of bacteria. They include members like mycobacterium, corynebacterium, streptomyces, and actinomyces. The diameter of an actinomycete cell is much smaller (1-2 µm) than the branched fungi (ranges between 5 µm-10 µm).
The filamentous forms of actinomycetes are aerobic and they may produce spore singly or in chains. Its colony appears as a pigmented powdery mass due to the formation of aerial spores.
Actinomycetes share the following physicochemical properties::
- Actinomycetes usually have 1-2 µm diameter.
- They generally possess a rod shape with a filamentous or branched structure. The filaments contain mumaric acid.
- Most of the species are aerobic, while a few are anaerobes to facultative aerobes.
- Cell wall and internal structures are similar to bacteria. The cell wall of actinomycetes consists of mycolic acid.
- The growth or reproduction of actinomycetes is slower than the bacteria and fungi. Hence, actinomycetes are sometimes called as “Slow growers”.
- They are having 60-78% of G+C content.
- Actinomycetes are most abundant in soil (106-108g) and marine habitat.
- The majority of species are usually non-motile, non-capsulated and non-acid fast.
- They show optimum growth at alkaline pH.
Life Cycle of Actinomycetes
Their life cycle includes the following stages:
- Germination: The spores of actinomycetes remain dispersed in the environment as a “Free spore”. Free spore remains dormant until the stage of germination. When the spores get favourable conditions, they start the germination process by forming a germ tube.
- Vegetative growth: The germ tube promotes vegetative growth, which eventually gives rise to the substrate and aerial hyphae. First, a germ tube will produce a primary mycelium, i.e. substrate hyphae that grow within the media. After the growth of primary mycelium, a secondary mycelium, i.e. aerial hyphae forms above the substratum.
- Coiling: During unfavourable conditions, the aerial hyphae turn into a spiral shape.
- Septation: At this stage, a septum forms between the vegetative hyphae.
- Spore maturation: A septum within the vegetative hyphae maturates and forms a chain of spores. Thus, the spores originate via fragmentation or swelling of the hyphae.
- Release of spore: During unfavourable conditions, the spores detach from the vegetative hyphae and remain free in the environment.
Actinomycetes have seven families, based on the hyphal and reproductive structures.
- Streptomycetaceae: Members of this family consist of non-segmented hyphae and 5-50 conidial spores per chain of aerial hyphae. Examples: Streptomyces, Microdlobaspone and Sporoctilhya.
- Nocardiaceae: Members of this family possess typical non-segmented hyphae. Examples: Nocardia, Pseudonocardia.
- Micromonosporaceae: Members of this family possess typical non-segmented conidia that generally exist singly, in pairs or in chains. Examples: Micromonospora, Thermonospora, Thermoactinomycetes, Actinobifida.
- Actinoplanaceae: Members of this family have sporangiospores and hyphae (0.2-2.0 µm wide). Examples: Streptosporangium, Actinoplanes, Plasmobispora and Dactylosporangium.
- Dermatophilaceae: Members of this family possess hyphae that undergo fragmentation to give rise to a large number of motile structures. Examples: Geodermatophilus.
- Frankiaceae: Members of this family are strictly associated with the roots of a non-leguminous plant and help in nitrogen fixation by forming root nodules. Example: Frankia.
- Actinomycetaceae: The members of this family do not contain true mycelium and are facultative anaerobes. Examples: Actinomyces.
Actinomycetes are the economically important organisms that play a fundamental role in many areas like:
Use in Bioremediation
Actinomycetes digest complex carbohydrates like chitin, cellulose, hemicellulose etc. It also helps in the degradation of toxic compounds from the environment. Thus, it plays an essential role in the bioremediation of organic compounds. Actinomycetes can survive in a harsh environment like high temperature up to 50 degrees Celsius that is crucial for the composting process.
Members of actinomycetes can produce many of the best-known antibiotics like amphotericin, neomycin, novobiocin, chloramphenicol, tetracycline etc.
- Tetracycline and erythromycin etc. target bacterial ribosomes and cures respiratory infections.
- Vancomycin mainly attacks the bacterial cell wall of pathogenic bacteria (Streptococcus aureus).
- Rifampicin targets bacterial RNAP (RNA-Polymerase) and cures tuberculosis and leprosy.
- Adriamycin treats cancer.
- Amphotericin attacks fungal membranes and shows a few side effects.
- Rapamycin enables organ transplant.
Use in Regulating Plant growth
Actinomycetes inhabit the soil and produce phytohormones, extracellular enzymes and bioactive compounds. These compounds promote direct plant growth and protect against phytopathogens and pests by producing indole 3-acetic acid, siderophore and solubilize phosphate.
Actinomycetes produce several enzymes, which show a wide range of applications in different fields like:
- Lipase in detergent and pharmaceuticals industries.
- Cellulases in the animal feed industry.
- Catalase in the detergent industry.
- Amylase in food, textile and paper industries.
- Chitinase in biochemical industries.
Use as Agroactive compounds
Actinomycetes produce agroactive compounds as they are extensively present in the rhizospheric zone of the plant. Thus, they can actively colonize themselves with the plant roots and protect the plant from pathogenic fungi and other phytopathogens. Frankia is an example of actinomycetes, which acts as a “Symbiont” that promote root nodule formation and thereby in nitrogen fixation.
Actinomycetes produce secondary metabolites, which act as AMSs (Antimicrobial substances). These antimicrobial substances attack pathogenic and phytopathogenic microorganisms, which can cause biocorrosion.
Use as Biopesticide
Actinomycetes are also used as a biopesticide that attacks insects like Musca domestica, Culex quinquefasciatus etc. It kills 90% of insects at their larval and pupal stage.