Bryant and Burkey Medium, Granulated

Principle and Interpretation

Bryant and Burkey Medium is used to enumerate the spores of gasogenic clostridia that are responsible for swelling of cheese in dairy industry (1, 2). In normal conditions of use, the medium allows the growth of other microorganisms also which are not directly related to cheese alteration, e.g. Clostridium sporogenes or Clostridium butyricum. Germination and growth of butyric acid bacteria (BAB) causes severe defects in cheese with silage being the main source of BAB spores in cheese milk (4). Clostridia spores are heat resistant and therefore can contaminate cheese brines. The gas produced by the growth of clostridia swells the cheese and is responsible for defect known as butyric swelling, resulting in bad taste. The main species causing this butyric swelling defect is Clostridium tyrobutyrium

Recommended technique for estimation of Clostridium is to enumerate the spores by the MPN method. Test sample must be previously decontaminated by heating up for 10 minutes at 75°C in order to destroy all the vegetative forms while only leaving the spores alive. The tubes of the medium are then boiled to regenerate anaerobiosis and cooled down to 25-30°C. These tubes are inoculated with the sample and overlaid with 2 cm of sterile paraffin and incubated for upto 7 days at 37°C. Examine the tubes after every 48 hours. A raised paraffin plug indicates gas formation. Tubes will be declared positive if they show clear gas production and MPN index is used to calculate the number of Clostridia

Resazurin is a redox indicator and monitors the oxygen level turning from pink under aerobic conditions to colourless under anaerobic conditions. The nutrient composition of the basal medium, particularly of casein enzymic hydrolysate, yeast extract, beef extract and L-cysteine HCl help in rapid growth of Clostridium species. Sodium acetate promotes spore germination (3) and improves the selectivity of the medium. Sodium lactate is fermented under anaerobic conditions by Clostridium tyrobutyricum. A pink colour indicates the presence of oxygen in the media which turns colourless on boiling.