Phenylalanine Malonate Broth , Shaw and Clarke Medium

Principle And Interpretation

The term enteric bacteria (or "enterics") is generally used in reference to organisms of the Family Enterobacteriaceae , many members of which occur in the enteric tract of humans and animals. Members of Enterobacteriaceae are the most frequently encountered bacterial isolates recovered from clinical specimens. Definitive identification of the members of the Enterobacteriaceae may require a battery of biochemical tests (1). This medium is prepared according to the formulation developed by Shaw and Clarke (2) for differentiating gram-negative enteric bacteria on the basis of their ability to utilize malonate and produce pyruvic acid from phenylalanine (4).

Yeast extract in the medium supplies nutrients to the organisms while phosphates buffer the medium. Bromothymol blue is the pH indicator. Sodium chloride maintains osmotic balance. Organisms like Klebsiella and Salmonella arizonae, which are capable of, utilizing malonate, produce an alkaline reaction and thus change the colour of the medium from light green to dark blue due to the pH indicator bromothymol 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 deaminating phenylalanine to pyruvic acid. This reaction can be determined by the addition of few drops of 10% ferric chloride dissolved in acidified distilled water to a freshly grown culture. Deep green colour is formed due to production of pyruvic acid from phenylalanine. Malonate utilization results should be read before adding ferric chloride solution to the test tube, to detect phenylalanine deamination (3).