Malonate Phenylalanine Broth

This medium is prepared according to the formulation developed by Shaw and Clarke for differentiating Gram-negative enteric bacteria on the basis of their ability to utilize malonate and produce pyruvic acid from phenylalanine, which changes the color of the medium from green to blue due to the pH indicator bromothymol blue. Yeast extract supplies nutrients to the organisms while two phosphates buffer the medium. Bromothymol blue is the pH indicator. Sodium chloride maintains osmotic balance. Potassium phosphates act as a buffer system. Sodium chloride supplies essential electrolytes for transport and osmotic balance.

Organisms like Klebsiella and Salmonella arizonae, being capable of utilizing malonate, produce an alkaline reaction and thus change the colour of the medium from light green to dark blue. The colour of the medium remains light green if the organisms are malonate negative. Members of the group Proteus and Providencia are capable of forming pyruvic acid from phenylalanine. This can be determined by the addition of a few drops of 10% iron(II) chloride dissolved in acidified distilled water to a freshly grown culture. Deep green colour is formed due to production of pyruvic acid from phenylalanine.

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

Preparation

Suspend 11 grams of the medium in one liter of distilled water. Mix well and dissolve by heating with frequent agitation. Boil for one minute until complete dissolution. Dispense into appropriate containers and sterilize in autoclave at 121 °C for 15 minutes.

Instructions for use

Inoculate and incubate at 35 ± 2 °C for 18-24 hours.

• Ewing, W.H., Davis, B.R. and Reavis, R.W. 1957. Phenylalanine and Malonate Media and Their Use in Enteric Bacteriology. The Public Health Laboratory, 15:153.