Bacterial antibiotic production. Colony of Streptomyces coelicolor bacteria growing on an agar medium (brown) 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 the antibiotic undecylprodigiosin (red areas), which has diffused into the surrounding medium. Antibiotic production is associated with differentiation of the bacterial colony into spore-producing structures (the thickened pitted surfaces).