A heterogeneous collection of bacteria with diverse ecologies and physiologies. The initial definition was based on their branching filamentous cellular morphology during a stage of the growth cycle. The name actinomycete means “ray fungus,” and for many years the actinomycetes were erroneously considered to be fungi, or at least closely related to the fungi. True fungi, such as the common bread mold, also grow as branching filaments (hyphae) that are several micrometers in diameter, whereas those of the actinomycetes are about 1 μm in diameter. The genetic material of the eukaryotic true fungi consists of DNA associated with proteins, and is enclosed in a membrane (the nucleus), whereas the genetic material of the prokaryotic actinomycetes consists of free DNA in direct contact with the cell sap (cytoplasm). In general, actinomycetes do not have membrane-bound organelles, whereas the fungi have an array of organelles such as mitochondria and vacuoles. The cell walls of fungi are made of chitin or cellulose, whereas the cell walls of the actinomycetes are made of a cross-linked polymer containing short chains of amino acids and long chains of amino sugars. Actinomycetes are susceptible to a wide range of antibiotics used to treat bacterial diseases, such as penicillin and tetracycline.