Bacterial antibiotic production. Colony of Streptomyces coelicolor bacteria growing on an agar medium (grey) in a petri dish. Streptomycetes are filamentous soil bacteria and a valuable source of antibiotics. Over half of Streptomycetes bacteria isolated from soil produce antibiotics. Here, a mature colony of a strain of S. coelicolor has produced two antibiotics: undecylprodigiosin (red-pink areas) and actinorhodine (intense blue areas). Actinorhodine is not used clinically, while undecylprodigiosin is used as an immunosuppressive agent.