Actinomycete comprises a group of Gram-positive, slow-growing, pleomorphic and rod-like anaerobic bacteria that form spores and mycelium (i.e. the branching filaments or hyphae of fungi). They are fungi-like filamentous bacteria. Mycelium formation in actinomycetes usually occurs during the growth of the bacteria, and is scarcely seen in old cultures. Actinomycetes are important normal microflora of the gastrointestinal tract (GIT) of humans and animals; and some are also found in the oral cavity of humans as microflora. They are widespread, and are naturally found in aquatic habitat, composts and in the soil. Morphologically, actinomycetes resemble corynebacteria and mycobacteria. Actinomycetes are distinct from other bacteria species or genera in that they are filamentous organisms, and they also form conidia (spores) and reproduce by asexual reproduction. Most actinomycetes are non-motile organisms. Some of the key genera of actinomycetes include: Nocardia, Actinomyces, Streptomyces, Rhodococcus and Actinomadura.
Nocardia species are aerobic and spore-forming branching bacilli normally found in the soil as saprophytes, and they are implicated as causative agent of nocardiosis in man. Actinomyces species are anaerobic or microaerophilic branching bacilli normally found in the soil, oral cavity, female genitalia and GIT as normal flora, and they are implicated as causative agent of actinomycosis. Streptomyces species are aerobic branching bacilli found naturally in the soil as saprophytes. They rarely cause human disease. Several species of the genus Streptomyces are of industrial and medical importance because they synthesize a wide variety of antibiotics used to treat bacterial infections. For example, tetracycline, streptomycin, clindamycin, neomycin, nystatin and amphotericin B are synthesized by S. aureofaciens, S. griseus, S. lincolnensis, S. fradiae, S. noursei and S. nodosus respectively. Rhodococcus species are aerobic cocci-bacilli organisms commonly found in the soil and in animals (e.g. horses); and they are implicated as causative agents of pneumonia in man. Actinomadura species are aerobic branching bacilli that cause mycetoma in humans.
IMPORTANCE OF ACTINOMYCETES
Actinomycetes are opportunistic bacteria with much resemblance to fungi organisms because of their ability to form long-branching filaments (mycelium) and conidia or spores. They occur naturally in the soil and in other habitats, and possess some of the following benefits:
- They are saprophytic organisms.
- They are the primary source of most naturally synthesized antibiotics used clinically to treat infectious diseases (e.g. streptomycin and tetracyclines).
- They are human and animal pathogens.
- Some species also cause disease in plants.
- They play significant roles in the mineralization of organic matters.
- Actinomycetes produce secondary metabolites that are of immense importance in the industry, medical and pharmaceutical companies.
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