Actinomycetes are classified as a group of gram-positive bacteria that are unique on their spore forming abilities and formation of mycelia structures. Actinomycetes refer to the organisms which are morphologically transitional between bacteria and fungi but yet possessing distinctive features to delimit them into a distinct category.though they are placed together with bacteria taxonomically in class Schizomycetes and in order Actinomycetales
Characteristics similar to Fungi are:
- They possess extensive branching mycelium.
- Aerial mycelium and asexual spores (conidia) on hyphae.
- Growth in broth media results pellets/clumps not turbidity of medium.
Characteristics similar to Bacteria are:
- Staining reaction and physiology are similar to bacteria.
- Cell size similar to bacteria. The diameter of hyphae rarely exceeds 1µ.
- Some species have flagella similar to bacteria.
- Cell wall composition is similar to bacteria and absenceof chitin and cellulose.
- Susceptible to antibacterial agents but not antifungal agents.
They are unicellular like bacteria, but produce a mycelium which is non-septate (coenocytic) and more slender, tike true bacteria they do not have distinct cell-wall and their cell wall is without chitin and cellulose (commonly found in the cell wall of fungi).
On culture media unlike slimy distinct colonies of true bacteria which grow quickly, actinomycetes colonies grow slowly, show powdery consistency and stick firmly to agar surface. They produce hyphae and conidia / sporangia like fungi. Certain actinomycetes whose hyphae undergo segmentation resemble bacteria, both morphologically and physiologically.
They are also known as ray fungi since actinomycetes are prokaryotic organisms with filamentous nature, branching pattern, and conidia formation, which are similar to those of fungi.
Actinobacteria produce branching mycelium of two kinds, Substrate and Aerial mycelium. These are filamentous bacteria with high G+C content i.e. About 55% of guanine and cytosine in their DNA.
Actinomycetes are numerous and widely distributed in soil and are next to bacteria in abundance. They are widely distributed in the soil, compost etc. Plate count estimates give values ranging from 10^4 -10^8 per gram of soil. They are sensitive to acidity / low PH (optimum PH range 6.5 to 8.0) and waterlogged soil conditions.They are heterotrophic, aerobic and mesophilic (25-30)0C organisms while some species commonly present in compost and manures are thermophilic growing at (55-65) 0C temperature (eg. Thermoatinomycetes, Streptomyces).
Actinomycetes are grouped under four families viz Mycobacteriaceae, Actinomycetaceae, Streptomycetaceae and Actinoplanaceae. Actinomycetous genera which are agriculturally and industrially important are present in only two families of Actinomycetaceae and Strepotmycetaceae.
In the order of abundance in soils, the common genera of actinomycetes are Streptomyces (nearly 70-90 %), Nocardia (10-30%) and Micromonospora although Actinomycetes, Actinoplanes, Micromonospora and Streptosporangium are also generally encountered.
Some important genera of actinomycetes are Streptomycetes, Nocardia, Micromonospora, Thermomonospora, Actinoplanes, Microbispora, Streptosporangium, Actinomadura, Actinosynnema, Dactylosporangium, Rhodococcus, Actinosynnema Kitasatospora, Gordona, Intrasporangium and Streptoalloteichus.
Functions / Role of actinomycetes:
These are inhabitants of soil that play important role in decomposition, recycling of minerals, composting, humus formation as well as production of antibiotics. Actinomycetes are also reported to cause some type of diseases. Some major role of actinomycetes includes:
- They are responsible for earthy / musty odor / smell of freshly ploughed soils.
- Streptomyces scabies,actinomycets strain causes “Potato scab” disease in potato.
- Degrade/decompose organic substances like cellulose, polysaccharides, protein fats, organic-acids etc.
- They decompose / degrade the more resistant and indecomposable organic substance/matter and produce a number of dark black to brown pigments which contribute to the dark colour of soil humus.
- They are also responsible for subsequent further decomposition of humus (resistant material) in soil.
- Many actinomycetes are noted as potential source of antibiotics. E.g. Streptomyces produce/synthesize number of antibiotics like Streptomycin, Terramycin, Aureomycin etc.
About 22,500 biologically active compounds obtained from microbes till today, 45% are from actinomycetes, 38% from fungi and 17% from other bacteria. The most important sources of antibiotics are Streptomyces and Micromonospora. It has been stated that Streptomyces account for about 80% of the total antibiotic products; while Micromonospora closely follows with less than one tenth as much as Streptomyces.
Organic residues / substances added to soil are first attacked by bacteria and fungi and later by actinomycetes, because they are slow in activity and growth than bacteria and fungi.
Antibiotics such as Tetracyclines, Aminoglycosides (Streptomycin and its relatives), Chloramphenicol, Rifamycin, Macrolides (Erythromycin and its relatives) etc. and most other clinically useful antibiotics are produced by actinomycete, Streptomyces species
Actinomycosis is the condition of infection of human and animals by actinomycetes. Various Actinomyces and Nocardia species are involved in Actinomycosis. It is a chronic granulomatous condition which commonly manifest as cervicofacial, pulmonary or abdominal disease caused by slowly progressive infection with oral and gastrointestinal communal Actinomyces israelii. Nocardiosis (Nocardia infections) are almost exclusively seen in immunocompromised patients.