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.