Differentiation of Clostridium perfringens from Related Clostridia in Iron Milk Medium

The stormy fermentation reaction of Clostridium perfringens in iron milk medium was compared to that of several C. perfringens -like strains. These clostridia, C. barati , C. perenne , C. absonum , and C. paraperfringens are very similar to C. perfringens on the basis of certain biochemical reactions and, consequently, are often difficult to distinguish from C. perfringens . Furthermore, these related clostridia may also be present in foods. Results of this study demonstrate that after 18 h of incubation at 45°C, only C. perfringens gave a positive reaction in iron milk with inocula as low as 22 cells/g. Some of the other strains began to show only gas production at 18 h. After 24 to 42 h some strains gave positive results and after 72 h all were positive. Enumeration of C. perfringens from food samples in iron milk medium by a 3-tube most probable number (MPN) technique gave similar results to enumeration by plate count using Shahidi-Ferguson Perfringens (SFP) agar. Furthermore, a rapid positive response occurred after only 2 and 3 h incubation of iron milk inoculated with 10⁸ and 10⁷ cells/ml, respectively. The high selectivity, ease of identification and rapid growth of C. perfringens in iron milk make the iron milk MPN procedure a valuable assay for accurate enumeration and differentiation of C. perfringens from related Clostridia in food products.