IRON MEDIUM BASE

Clostridium is a large genus of gram-positive spore bearing anaerobes. Clostridium perfringens is one of the most common anaerobes found in foods. Small numbers of C. perfringens are commonly found in raw meats, poultry, dehydrated soups and sauces, raw vegetables and spices. The spores of these strains are resistant to high temperatures and survive at 100?C for more than one hour. Inadequately processed foods and improper storage often leads to proliferation of these organisms. Hence detection of C. perfringens become necessary. Iron Milk Medium is one of the medium for presumptive detection of C. perfringens in accordance with FDA, BAM. On isolation of black colonies from suspected foods on TSC agar, the culture is enriched in Fluid thioglycollate medium. The enriched culture is tested for stormy fermentation in Iron milk Medium Base with added whole milk.
As per the procedure, the food sample under test; whole portion or representative 25 gm is checked for total bacterial count by inoculating on TSC agar. Presumptive Clostridia species grow as black colonies which is cultured and enriched in Fluid Thioglycollate Medium at 35?C for 18-24 hours. Inoculate modified iron-milk medium with 1 ml of actively growing C. perfringens in Fluid Thioglycollate culture and incubate at 46?C in a water bath. Make periodic observations after 2 hours for “stormy fermentation?, which is characterized by rapid coagulation of milk followed by fracturing of curd into spongy mass which usually rises above medium surface. Bigger test tubes are used for the prevention of spillage into the water bath. Cultures that fail to exhibit “stormy fermentation” within 5 h are unlikely to be C. perfringens. An occasional strain may require 6 h or more, but this is a questionable result that should be confirmed by further testing. Some strains of C. baratii react in this manner, but this species can be differentiated by its inability to liquefy gelatin in lactose-gelatin medium. The rapidity with which the “stormy fermentation” occurs depends on the strain and the initial population. Therefore, only actively growing cultures are appropriate for this test. The presumptive test in iron-milk medium may be sufficient for some purposes. However, the completed test must always be performed with isolates associated with food poisoning outbreaks.

for presumptive con?rmation of Clostridium perfringens from food in accordance with FDA BAM, 1998