Bacterial antibiotic production. Colonies of Streptomyces coelicolor bacteria growing on an agar medium (grey) in a petri dish held by a laboratory researcher. Streptomycetes are filamentous soil bacteria and a valuable source of antibiotics. Over half of Streptomycetes bacteria isolated from soil produce antibiotics. Here, five different strains of S. coelicolor have been cultivated, with some producing faint traces around the colony edges of one or more of two antibiotics: undecylprodigiosin (red-pink) and actinorhodine (blue). Actinorhodine is not used clinically, while undecylprodigiosin is used as an immunosuppressive agent.