Gram-positive bacteria rival the proteobacteria in diversity. Species in one subgroup, the actinomycetes form colonies containing branched chains of cells. Two species of actinomycetes cause tuberculosis and leprosy. Most actinomycetes are free living species that help decompose the organic matter in soil; their secretions are partly responsible for the "earthy" odor of rich soil. Their secretions are partly responsible for the "earthy" odor of rich soil. Soil-dwelling species in the genus Streptomyces are cultured by pharmaceutical companies as a source of many antibiotic, including streptomycin. Gram-positive bacteria include many solitary species, such as Bacillus anthracis, which causes anthrax, and Clostridium botulinum, which causes botulism.