Actinomycetes

Actinomycetes

1.      Gram positive, filamentous, aerobic bacteria.

2.      The group has been placed separately in volume 4 of the Bergey’s Manual of Systematic Bacteriology.

3.      The name of the group is derived from the genus name Actinomycetes meaning ‘ray-fungi’ (Gr. Action – rays, mykes- fungus)

4.      They are basically soil inhabitants. However few types of actinomycetes are found in the body cavities of man and animals.

5.      On solid substratum, the branching network of hyphae developed by actinomycetes grow both on the surface of the substratum called aerial hyphae and inside the substratum called  substrate mycelium.

6.      Septa usually divide the hyphae into long cells containing several nucleiods.

7.      Many actinomycetes have aerial mycelium that extends above the substratum and form asexual, thin walled spores, conidia or conidiospores on the ends of filaments.

8.      The mycelial structure and the formation of aerial branches and spores give these organisms the appearance of fungi

9.      Most actinomycetes are non-motile, when motility is present it is confined to flagellated spores.

10.  The cell wall composition of actinomycetes varies greatly among different groups.

11.  Four major groups cell types can be distinguished according to three feature of peptidoglycan composition and structure i. the amino acid in tetrapeptide side chain postion, ii. The presence of glycine in interpeptide brides, and iii. Peptidoglycan sugar content.

12.  Reproduction: Actinomycetes, like other bacteria, reproduce only asexually. The asexual mode of reproduction is accomplished by arthrospore or conidia or conidiospore.

13.  Arthrospore formation: the filamentous bodies of the actinomycetes break into rod-shaped smaller fragments called arthrospores. Each is capable of growing into a new filament.

14.  Conidia formation: Condia formation is a common method of reproduction in some members of actinomycetes. The filamentous branched actinomycetes produce smaller, oval or rounded structures called conidia terminally on certain apical branches called conidiophores. Each conidium germinates giving rise to a actinomycetes cell. Ex. Streptomyces.

Economic Importance

Producers of Antibiotics

More than 500 antibiotics have been obtained from different species of streptomycetes

Example:

            Streptomycin – Streptomyces griseus

            Neomycin      - S. fradiae

            Tetracycline  - S. aureofaciens

            Erythromycin – S. erythreus

            Clindamycin   - S. lineolnesis

             Chloramphenicol – S. venezuelae

In Agriculture:

As a agent of Biological control

There are innumerable reports of actinomycetes with activity against plant pathogens or reports concerning the prevelance of antagonistic actinomycetes in the rhizosphere of diseased plants.

It has been currently reported that addition of cellulosic waste products (rice stubble or water hyacinth biomass) results in the reduction of cauliflower damping off by Rhizoctonia solani. This reduction has been ascribed to the stimulation of antagonistic actinomycetes.

Spores of Phytopthora have been shown to be parasitized by species of Actinoplanes, Amulariella. This phenomenon is called hyper parasitism.

As a enhancers of plant growth

Some free living actinomycetes have been shown to be indirectly involved in production of Vitamin B in Pine rhizosphere. Since mycorrhiza require these vitamins, actinomycetes are indirectly involved in plant growth enhancement.

As producers of agriculture chemicals

Actinomycetes are responsible for antibiotics used in agriculture as agents of control of bacterial and fungal diseases and insects pests.

Drugs having insecticidal value are Monensin, salinomycin.

Nutritional antibiotics are given to farm animals for better growth and assimilation of feed are moenomycin and elfacins.

The antifungal antibiotics used in plant disease control derived from streptomyces  include cyclohexamide, blasticidin (against rice blast pathogen), polyoxins (against Alternaria spp)

Mineralisation of organic matter

Actinomycetes can degrade enormous number and variety of organic compounds and are extremely important in the mineralization of organic matter.

Harmful

As animal pathogen

Several members of actinomycetes are pathogens of cattle and farm animals

Corynebacterium pyogenes causes mastitis, pharyngitis and urethrites in sheep, cows, swine and horses. Mycobacterium farcinogenes causes subcutaneous inflammation of lymph nodes and vessels in cattle.

As Human pathogen

They  are causative agents of a few human diseases  such as actinomyosis, various abscesses.