Actinomycetes are gram-positive organisms whose filamentous,
hyphalike appearance was responsible for their earlier, erroneous
classification; they are in fact bacteria that are abundant in compost,
soils, food, and freshwater and have been isolated from the
atmosphere. The most important species implicated in
hypersensitivity pneumonitis include Micropolyspora faeni,
Thermoactinomyces vulgaris, T. viridis, and T. candidus.