A major group of gram-positive bacteria includes Actinobacteria. This phylum is commonly found in soil and plant materials and contains primarily aerobic bacteria that are rod shaped to filamentous. The Actinobacteria is mostly made of harmless commensals except for Mycobacterium tuberculosis. Therre are nine orders in Actinobacteria, but most belong to Actinomycetales. Coryneform bacteria belongs to the Actinomycetales. These bacteria are gram-positive, aerobic, and nonmotile. They from irregularly shaped cells, club-shaped or V-shaped cells that are a result of snapping division.

An important branch of the Actinobacteria is the Mycobacterium. This genus contains several important human pathogens, including tuberculosis. This species is rod-shaped and possess a distinctive property called acid-fastness. Mycolic acids, unique to Mycobacterium is responsible for a waxy surface around the cell. This waxy surface causes the cell not to stain well with the gram stain.

Mycobacteria may undergo branching or filamentous growth, but the filaments do not form a true mycelium. Mycobacteria are separated into the slow growing species and the fast growing species with tuberculosis belonging to the slow growing species. Most mycobacteria grow aerobically in mineral salts and use ammonium as its nitrogen source and glycerol or acetate as their sole carbon source and electron donor. Tuberculosis is different and requires fatty acids and lipids.

Another important species of Actinobacteria is the Streptomyces. The Streptomyces are a filamentous species.  Filaments form branching hyphae that then form into mycelium. Streptomyces also form spores called conidia that are distinct from endospores made by Bacillus and Clostridium. Streptomyces is primarily found in the soil. Another important fact is that Streptomyces produce antibiotics.