Phenol Red Dextrose Broth

Casein peptone provides nitrogen, vitamins, minerals and amino acids essential for growth and allows the abundant growth of a wide variety of fastidious microorganisms. Sodium chloride supplies essential electrolytes for transport and osmotic balance. Phenol red is the pH indicator and changes to yellow in acid conditions as a result of bacterial fermentation. Durham tubes trap any gases produced during fermentation. Dextrose is the fermentable carbohydrate providing carbon and energy. Vera recommended using casein peptone in fermentation test media since she found that it could be used with the pH indicator phenol red in fermentation tests with a high degree of accuracy.

Typical formula in g/L, adjusted and/or supplemented as required to meet performance criteria.

Preparation

Suspend 20 grams of the medium in one liter of distilled water. If the medium is for the cultivation of anaerobes, add 0.5-1 g of agar. Mix well and dissolve by heating with frequent agitation. Boil for one minute until complete dissolution. Dispense in amounts of 5 mL into tubes with Durham gas collecting tubes for gas detection. Sterilize in autoclave at 118 °C for 15 minutes. DO NOT OVERHEAT.

Instructions for use

1. Inoculate tubes with test organisms and incubate at 37 °C for 18-48 hours.
2. Observe for color change. The appearance of a yellow color is the indication of fermentation, with or without gas production.
3. Control tubes of the uninoculated medium should be run parallel with inoculated tubes.
4. Tubes should be examined frequently because different carbohydrates are utilized at variable speeds.

For anaerobes, the medium should be used on the day of preparation. If not, the medium must be heated and cooled before use.

• Rogers, Ryan and Severans. Antibiotic and Chemother 5:382. 1955.
• Association of Official Analytical Chemists. 1995 official methods of analysis of AOAC Arlington, VA.
• Baron EJ LR Peterson and S.M. Finegold 1994. Bailey & Scott's diagnostic microbiology, 9th edition. Mosby-Year Book, Inc. St. Louis, MO.
• Murray, PR., E.J. Baron M.A. Pfaller F.C. Tenover and R.H. Yolken (ed) 1995. Manual of clinical microbiology, 6th edition. American Society for Microbiology, Washington DC.