Actinomycetes are a group of prokaryotic unicellular and gram-positive bacteria with high G+C content in their DNA.
These are mostly aerobic (but some members of the group are anaerobic also).
The actinomycetes form branched filamentous hyphae, which closely resembles with fungi hyphae. But their hyphal diameter is approx. 1µm, whereas in fungi it is 5 to 10µm.
These organisms reproduce by asexual spores, which are termed conidia when they are naked or sporangiospores when enclosed in a sporangium. Although these spores are not heat-resistant, they are resistant to desiccation and aid in survival of the species during periods of drought.
The actinomycetes are mainly harmless soil organisms, although a few are pathogenic for humans (Streptomyces somaliensis causes actinomyctoma of humans), other animals (Actinomyces bovis causes lumpy jaw disease of cattle), or plants (Streptomyces scabies causes common scab in potatoes and sugar beets).
In soil, they are saprophytic and chemo-organotrophic, and they have the important function of degrading plant or animal residues.
Also, some are best known for their ability to produce a wide range of antibiotics useful for treating human diseases. These organisms secrete extracellular enzymes, which are decomposers of dead organic material. These enzymes lyse bacteria and thereby keep the bacterial population in check and thus help to maintain the microbial equilibrium of the soil.
The Actinomycetes superficially resemble fungi for having subterranean and aerial hyphae and chains of spores, but their hyphal diameter, cytology, and chemical composition of cell walls are quite decidedly bacterial in pattern.
Somatic Structure of Actinomycetes
Most of the actinomycetes are mycelioid.
They begin their development as unicellular organisms but grow into branched filaments or hyphae, which grow profusely by producing further branches constituting the mycelium.
The width of the hypha is usually 1µm.
The delicate mycelia often grow in all directions from a central point and produce an appearance that has been compared with the rays of the sun or of a star. Therefore, the Actinomycetes are also called “ray fungi”.
The protoplasm of the young hyphae appears to be undifferentiated, but the older parts of the mycelium show definite granules, vacuoles, and nuclei.
Many Actinomycetes at first produce a very delicate, widely branched mycelium that may embed itself into the soil or, if grown in culture, into the solid medium. This kind of mycelium is therefore called the substratum or primary mycelium.
After a period of growth, hyphae of a different kind develop, which raise themselves up from the substratum mycelium and grow towards the air. These are called aerial hyphae, and the corresponding mycelium is the aerial or secondary mycelium. The aerial mycelium may be white, yellow, violet, red, blue, green, or grey and many form pigment that are excreted into the medium.
The aerial mycelium is usually slightly wider than the substratum mycelium.
The aerial hyphae possess an extra cell wall layer (sheath). The hyphal tip undergoes septation within its sheath to form a chain of conidia. Conidial cell contains a plump, deeply staining, oval or rod-shaped nuclear body.
Figure: Diagrammatic structure of Actinomycetes
Distribution and Mode of Nutrition of Actinomycetes
The actinomycetes are essentially mesophilic and aerobic in their requirements for growth and thus resemble both bacteria and fungi.
They, along with other microorganisms, form the soil microflora and produce powerful enzymes by means of which they are able to decompose organic matter.
The majority of these are soil organisms and are associated with rotting material.
The characteristic odour of soil after it is ploughed or wetted by rain is largely due to the presence of the Actinomycetes.
Some are pathogens.
The actinomycetes are particularly abundant in forest soil because of the abundance of organic matter. They occur mainly in soils of neutral pH, although some prefer acidic or alkaline soil.
The actinomycetes can grow in soils having less water content than that needed for most other bacteria.
Most of the actinomycetes are quite proteolytic and attack proteins and polypeptides, and are also able to utilize nitrates and ammonia as sources of nitrogen.
Nearly all synthesise vitamin B₁₂ when grown on media containing cobalt salts.
