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Principle
- Malonate broth is formulated by Leifson (1933) consists of yeast extract, ammonium sulfate, dipotassium phosphate, potassium dihydrogen phosphate, sodium chloride, sodium malonate, dextrose, and bromothymol blue.
- The media is developed to differentiate enterobacter from Escherichia species based on their ability to utilize malonate. The yeast extract provides nitrogen, carbon, vitamins, and other essential elements. Ammonium salt is a synthetic source of nitrogen.
- Phosphate acts are buffering agents. Sodium chloride maintains osmatic equilibrium. Dextrose is a carbon source. Bromothymol blue is a pH indicator dye. The medium, therefore, will support the growth of organisms that cannot utilize malonate or ammonium salt.
- The spontaneous alkalinization produced by organisms is buffered by the phosphate system and neutralized by the acid produced by the fermentation of a small amount of dextrose.
- An alkaline reaction (blue color) is produced in this medium by organisms capable of utilizing malonate and ammonium sulfate. The alkali changes the color of the bromothymol blue indicator in the medium to light blue and finally to Prussian blue.
- The color of the medium remains unchanged in the presence of an organism that cannot utilize these substances. Some malonate-negative strains produce a yellow color due to the fermentation of dextrose only, which results in increased acidity causing the pH indicator to change to yellow at a pH of 6.0.
- Also, some malonate-positive organisms produce only a slight alkalinity that causes the results to be difficult to interpret. Consequently, these tubes should be compared with an un-inoculated malonate tube.
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