Subclass Acidimicrobidae
    Acidimicrobiales
Subclass Actinobacteridae
    Actinomycetales
    Bifidobacteriales
Subclass Coriobacteridae
    Coriobacteriales
Subclass Rubrobacteridae
    Rubrobacterales
Subclass Sphaerobacteridae
    Sphaerobacterales
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. Few forms are pathogens, such as Mycobacterium. All are aerobic. There is an exception to this and is found in most microbial handbooks: Not all of this strain of bacteria, Actinobacteria, are aerobic. Actinomyces isrealii, in particular, is a slightly anaerobic gram positive rod found usually as oral flora (but can spread) and produces sulfer granules in anaerobic condition.
The Actinobacteria correspond to the high G+C group of Gram-positive bacteria, forms whose DNA contains unusually large amounts of the bases guanine and cytosine. Representative genera include: