Sodium metabisulphite and ferric ammonium citrate are used as an indicator of sulphite reduction by Clostridium perfringens. The reduction of sulphite by C.perfringens produces black colonies and the egg yolk emulsion incorporated into the media detects the lecithinase activity of this bacteria. However not all strains produce lecithinase and therefore black lecithinase positive and black lecithinase negative colonies should be considered as presumptive C.perfringens.