A simple method for screening porphyrin secretion by colonies of Escherichia coli

Escherichia coli secretes porphyrins when exposed to 20 mM 1‐thioglycerol. A method was devised to isolate mutants, which do not excrete porphyrins in the presence of thiols. DEAE‐Sephadex beads were incorporated into plates growing colonies of E. coli . The secreted porphyrins on these plates formed a fluorescent halo around each colony, when the plates were viewed under long‐wave ultraviolet light. Mutants were found, that formed either halo‐less colonies or a colony with excess halo. This method of incorporating immobilized ion exchangers into plates may be useful in isolating non‐secretor or super‐secretor mutants of a variety of organisms.