The Actinobacteria are a group of Gram-positive bacteria. Most are found in the soil, and they include some of the most common soil life, playing important roles in decomposition and humus formation. Some form branching filaments, which somewhat resemble the mycelia of the unrelated fungi, among which they were originally classified as the Actinomycetes. All are aerobic. A few forms are pathogens, such as Mycobacterium. Some are thermophilic.