Actinomycetes: Economic Importance and Reproduction

In this article we will discuss about:- 1. Characteristics of Actinomycetes 2. Historical Review of Actinomycetes 3. Economic Importance 4. Distribution and Mode of Nutrition 5. Somatic Structures 6. Reproduction.

Contents:

1. Characteristics of Actinomycetes
2. Historical Review of Actinomycetes 
3. Economic Importance of Actinomycetes
4. Distribution and Mode of Nutrition of Actinomycetes
5. Somatic Structures of Actinomycetes
6. Reproduction of Actinomycetes

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1. Characteristics of Actinomycetes:

The Actinomycetes or Streptomycetes or Actinomycetales as they are called are a group or Gram-positive bacteria which form branched filamentous hyphae having resemblance with fungal hyphae. But their hyphal diameter is approximately 1µm, whereas in fungi it is 5 to 10 µm.

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 survival of the species during periods of drought.

These filamentous bacteria are mainly harmless soil organisms, although a few are pathogenic for humans (Streptomyces somaliensis causes actinomycetoma of human), 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 chemoorganotrophic, and they have the important function of degrading plant or animal resides.

Again some are best known for their ability to produce a wide range of antibiotics useful in treating human diseases. These organisms excrete 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.

2. Historical Review of Actinomycetes:

The early exploratory studies by McCormack (1935) and Alexopoulos and Herrick (1938-1942) were followed by the intensive studies by Professor S. A. Waksman and his students (1943-1951) which culminated in the discovery of streptomycin and other new and potentially useful chemotherapeutic agents.

Nearly 100 antibiotic substances have been reported in the literature as metabolites of the Actinomycetes. A few of these have been isolated in pure form and their chemistry studied in detail, while others have been described only as concentrates or in a preliminary way.

3. Economic Importance of Actinomycetes:

The Actinomycetes, forming soil micro-flora have gained the greatest importance in recent years as producers of therapeutic substances.

Many of the Actinomycetes have the ability to synthesize metabolites which hinder the growth of bacteria; these are called antibiotics, and, although harmful to bacteria are more or less harmless when introduced into the human or animal body. Antibiotics have in modern times great therapeutical and industrial value.

The past decade has seen considerable interest in the Actinomycetes as producers of antibiotic substances. The successful use in chemotherapy of streptomycin, chloromphenicol (Chloromycetin is the trade name of this substance), aureomycin and terramycin all metabolites of the Actinomycetes, has stimulated the search for new Actino­mycetes and new antibiotics among the Actinomycetes.

The genus Streptomyces is the largest and the most important one, antibiotically speaking.

4. Distribution and Mode of Nutrition of Actinomycetes:

The Actinomycetes are essentially mesophilic and aerobic in their requirements for growth and thus resemble both bacteiia 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 grow slowly and on artificial media produce hard and chalky colonies which smell decaying leaves 01 musty earth. They 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 others bacteria.

The Actinomycetes are capable of utilizing a large number of carbohydrates as energy sources when the carbohydrates are present in the media as sole sources of meta- bolizable carbon.

Most of the Actinomycetes are quire proteolytic and attack proteins and poly­peptides, and are also able to utilize nitrates and ammonia as sources of nitrogen. Nearly all synthesize vitamin B₁₂ when grown on media containing cobalt salts, and many are able to synthesize rather complex organic molecules which have antibiotic properties. The mechanism of synthesis of these substances is not understood.

5. Somatic Structures 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 hyphae is usually 1 µm. The delicate mycelia often grow in all directions from a central point and produce an appearance that has been compared with the rays of sun or of a star.

Therefore, the Actinomycetes are also called ‘ray fungi’. They often produce complicated designs and resemble some of the drawings in modern art exhibitions. They are Gram-positive. 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’(Fig. 338).

After a period of growth, hyphae of a different kind develop, which raise themselves up from the substratum mycelium and grow into the air. These ate 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 pigments that are excreted into the medium.

The aerial mycellium is usually slightly wider than the substratum mycelium. The aerial hyphae possess an extra ceil wall layer (sheath). The hyphal tip undergoes septation within this sheath to form a chain of conidia. Conidial cell contains a plump, deeply staining, oval or rod-shaped nuclear body.

6. Reproduction in Actinomycetes:

Most species reproduce by conidia which are developed in chains from the aerial hyphae. The chains may be straight, flexuous (wavy) or coiled to various degrees. The conidia bearing filaments are often spirally twisted. Sometimes the whole length of the aerial hypha, sometimes only its upper part is transformed into conidia.

Each conidium has a roundish nucleus and is surrounded by a firm outer wall. The conidial wall may be smooth, warty, spiny, or hairy.

The conidia can persist in the dry state for many years. Even the vegetative forms of the Actinomycetes are quite hardy and are able to adapt themselves to the changing soil conditions.

The conidia appear as a fine powdery coat on the surface of cultures. When the conidia have been scattered on the ground and conditions are favourable they germinate producing one to three or even occasionally four little germ tubes which give rise to mycelioid condition (Fig. 338).

The primary mycelium in some species commonly breaks up into small fragments called arthrospores, which often look like bacterial cells and which might easily be mistaken for the latter.