a bacteria culture medium , to be used in anaerobic condition . a metal complex enables oxidative polymerisation of an indoxyl derivative and a substrate containing an indoxyl derivative leading to an insoluble coloured compound . the metal complex , in particular ammonium ferric citrate , contains a concentration ranging between 0 . 3 and 0 . 9 mg / ml , preferably 0 . 6 mg / ml . the culture medium may include a substrate such as x - gal , at a concentration ranging between 10 and 500 mg / l .