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Actinomycete comprises a group of Gram-positive, slow-growing, pleomorphic and rod-like anaerobic bacteria that form spores and mycelium. Mycelium is the branching filaments or hyphae of fungi. Actinomycetes 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), mycelium and reproduce by asexual reproduction. Their ability to form some characteristics of fungi (e.g., mycelium) is the reason why actinomycetes are called fungi-like filamentous bacteria.

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 human population. 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 in human population. Streptomycesspecies 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 other bacterial infections in human and animal population. For example, tetracycline, streptomycin, clindamycin, neomycin, nystatin and amphotericin B are synthesized by Streptomyces 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 human population. Actinomadura species are aerobic branching bacilli that cause mycetoma in human population.       


Actinomycetes are opportunistic bacteria with much resemblance to fungi 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:

  • Actinomycetes are saprophytic organisms.
  • They are the primary source of most naturally synthesized antibiotics (e.g., streptomycin and tetracyclines) used clinically to treat infectious diseases.
  • Actinomycetes are human and animal pathogens.
  • Some species of Actinomycetes also cause disease in plants.
  • Actinomycetes 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.

Further reading

Brooks G.F., Butel J.S and Morse S.A (2004). Medical Microbiology, 23rd edition. McGraw Hill Publishers. USA.

Gilligan P.H, Shapiro D.S and Miller M.B (2014). Cases in Medical Microbiology and Infectious Diseases. Third edition. American Society of Microbiology Press, USA.

Madigan M.T., Martinko J.M., Dunlap P.V and Clark D.P (2009). Brock Biology of Microorganisms, 12th edition. Pearson Benjamin Cummings Inc, USA.

Mahon C. R, Lehman D.C and Manuselis G (2011). Textbook of Diagnostic Microbiology. Fourth edition. Saunders Publishers, USA.

Patrick R. Murray, Ellen Jo Baron, James H. Jorgensen, Marie Louise Landry, Michael A. Pfaller (2007). Manual of Clinical Microbiology, 9th ed.: American Society for Microbiology.

Wilson B. A, Salyers A.A, Whitt D.D and Winkler M.E (2011). Bacterial Pathogenesis: A molecular Approach. Third edition. American Society of Microbiology Press, USA.

Woods GL and Washington JA (1995). The Clinician and the Microbiology Laboratory. Mandell GL, Bennett JE, Dolin R (eds): Principles and Practice of Infectious Diseases. 4th ed. Churchill Livingstone, New York.

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