SPS Agar, Granulated

Principle And Interpretation

SPS (Sulphite Polymyxin Sulphadiazine) Agar was developed by Angelotti et al (1) based on the Wilson and Blair Medium and the medium described by Mossel et al (2, 3) for selective isolation and enumeration of C. perfringens from foods. The medium of Mossel et al included the use of Miller-Prickett tubes. The modified SPS Agar however obviates the inclusion of Miller-Prickett tubes.

Casein enzymic hydrolysate and yeast extract supply nitrogenous compounds, vitamin B complex and other essential growth nutrients to the growing C. perfringens. This organism reduces sulphite to sulphide which reacts with iron of ferric citrate to form a black precipitate of iron sulphide and hence the colonies appear black (4). Polymyxin B and sulphadiazine inhibit a wide variety of gram-positive and gram-negative bacteria (5). Few organisms found in food other than C. perfringens also form black colonies on this medium.

Prepare serial dilutions of the samples to be tested and inoculate onto SPS Agar using the pour plate technique. If desired, pour cover layers using about 5 ml medium. Incubate the plates anaerobically. Enumerate the black colonies. Presumptive black C. perfringens colonies should be confirmed by standard biochemical tests.