This monograph has been reviewed by members of the IUMS–ICFMH Working Party on Culture Media and given ‘Proposed’ status.

The medium was first developed by Wilson and Blair (1924) to enumerate Clostridium perfringens (B. welchii) in water. The name is now given to a number of formulations used for different purposes. Iron sulfite agar used for the enumeration of mesophilic sulfite-reducing clostridia has the same composition as tryptose sulfite cycloserine base (Harmon et al., 1971) but without the cycloserine (vide infra). For the enumeration of thermophilic sulfite-reducing organisms such as Thermoanaerobacterium (formerly Clostridium) thermosaccharolyticum and Desulfotomaculum nigrificans, a nutritionally poorer composition is used (Attenborough and Scarr, 1957; Donnelly and Graves, 1992). This is the medium described here. Both media utilise the ability of the genus Clostridia to reduce sulfite, which reacts with the iron citrate to form ferrous sulfide, staining the colonies black.