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Actinomycetes is a nontaxonomic term for a group of common soil microorganisms sometimes called "thread or ray bacteria." They are known for decomposing more resistant organic materials such as chitin, a complex sugar found in the outer skeleton of insects and elsewhere. Actinomycetes are simple prokaryotic organisms that are gram positive, have a high G+C base composition, and are mycelium (thread) forming. Nucleic materials (DNA) consist of a helical structure with a sugar backbone attached through the bases adenine – thymine (A+T) and guanine – cytosine (G+C). Compared with the DNA of other organisms, actinomycetes have a high percentage of guanine – cytosine bases. In growth habit, many actinomycetes resemble fungi but are smaller, and terms common to both are used to describe morphological features. The most common genus of actinomycetes in soil is Streptomyces that produces straight chains or coils of spores or conidia. At 1,000x magnification, the round conidia making up the coils can be seen in the video. More than one-half of the antibiotics used in human medicine, including aureomycin, chloromycetin, kanamycin, neomycin, streptomycin, and terramycin, come from soil actinomycetes. The smell of freshly turned soil is due to metabolic end products called geosmins that are produced by these organisms and move through soil as unseen volatiles.

Figure 1: Soil Actinomycetes (video with audio).

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