Dictionary:

actinomycete

  (ăk'tə-nō-mī'sēt', -mī-sēt')

Any of various filamentous or rod-shaped, often pathogenic microorganisms of the order Actinomycetales that are found in soil and resemble bacteria and fungi.

A heterogeneous collection of bacteria that form branching filaments. The actinomycetes encompass two different groups of filamentous bacteria: the actinomycetes per se and the nocardia/streptomycete complex. Historically, the actinomycetes were called the ray fungi and were thought to be related to the true fungi, such as bread molds, because they formed mats (mycelia) of branching filaments (hyphae). However, unlike the true fungi, the actinomycetes have thin hyphae (0.5–1.5 micrometers in diameter) with genetic material coiled inside as free DNA. The cell wall of the hyphae is made up of a cross-linked polymer containing short chains of amino acids and long chains of amino sugars. In general, actinomycetes do not have membrane-bound cell organelles. Actinomycetes are susceptible to a wide range of antibiotics that are used to treat bacterial diseases, such as penicillin and tetracycline. See also Amino sugar; Antibiotic.

Members of the genus Actinomyces are most often found in the mouth and gastrointestinal tract of humans and other animals. Actinomyces do not require oxygen for growth and are sometimes referred to as anaerobic bacteria. It is actually the requirement for elevated levels of carbon dioxide rather than the negative effect of oxygen that characterizes Actinomyces. When displaced from their normal sites within the mouth or gastrointestinal tract, Actinomyces may cause diseases in humans, such as lung abscesses, appendicitis, and lumpy jaw, which is also seen in cattle. Serious ulcers of the cornea of the eye have been caused by contact lens contaminated with saliva containing Actinomyces.

The nocardia/streptomycete complex constitutes a continuous spectrum of organisms from those most like true bacteria to those that are superficially most like fungi. The nocardiae represent the transition, having members that resemble the bacteria that cause diphtheria (Corynebacterium) and tuberculosis (Mycobacterium). Members of the genus Nocardia require oxygen for growth, are found in soil and water, and have the ability to use a wide range of organic material as a source of energy. A few species of Nocardia cause disease in humans. Nocardiae inhaled from the soil may cause a disease of the lungs similar to tuberculosis. A few species produce clinically useful antibiotics. The streptomycetes have long branching filaments and two types of mycelia. The cell walls are typical bacterial cell walls and do not contain the fatty acids found in nocardiae and mycobacteria. Streptomycetes require oxygen for growth, are found in soil and water, and have the ability to utilize a wide range of organic materials as nutrients. The streptomycetes are particularly important in degradation of dead plant materials in soil; the aroma of fresh soil and newly dug potatoes is actually due to streptomycetes. Streptomycetes do not produce disease in humans or animals and are best known for producing many clinically useful antibiotics, including streptomycin, tetracycline, and cephalosporin. There are many other genera of actinomycetes, defined on the bases of morphology, chemical composition of cell walls, or unique roles in nature. See also Bacteria; Diphtheria; Medical bacteriology; Soil microbiology; Tuberculosis.

Filamentous microorganisms that have been implicated in the formation of dental calculus and serve as a mode of attachment of dental calculus to the tooth surface. These microorganisms have also been found in pathologic lesions of the alveolar processes (actinomycosis).

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A mold-like bacterium (order Actinomycetales) occurring as elongated, frequently filamentous cells, with a branching tendency.