SPS Agar (Sulfite Polymyxin Sulfadiazine) is a moderately selective medium to recover Clostridium perfringens from fresh or preserved foods and food ingredients. The medium was modified by Angelotti, incorporating sulfadiazine and polymyxin B sulphate to the more recent Mossel formula for the recovery of Clostridium perfringens. Casein peptone provides nitrogen, vitamins, minerals and amino acids essential for growth. Yeast extract is a source of vitamins, particularly of the B-group. Ferric citrate and sodium sulfite are H2S indicators. C. perfringens reduces the sulfite to sulfide which in turn reacts with the iron and forms a black iron sulfide precipitate, seen as black colonies. Polymyxin B sulfate and sulfadiazine are inhibitors to organisms other than Clostridium spp. Bacteriological agar is the solidifying agent. A few microorganisms other than C. perfringens also grow on SPS Agar so it is best to perform a Gram stain and look for spores. Many common microorganisms are totally or partially inhibited, but if they develop, they generally do not form black colonies nor spores, nor do they reduce nitrate and are non-motile Gram-positive vegetative bacilli.
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