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Principle
- OF Basal medium is developed by Hugh and Leifson (1953) to study oxidative and carbohydrate fermentative metabolism of gram-negative bacteria. The media is composed of tryptone, sodium chloride, Dipotassium hydrogen phosphate bromothymol blue, and agar.
- Tryptone provides nitrogen, vitamins, and necessary essential elements. Sodium chloride maintains osmatic equilibrium. Phosphates buffer the medium and bromothymol blue acts as a pH indicator.
- The low concentration of agar facilitates the determination of motility and enables to visualization of acid production at the surface of the medium. The motility is determined by the diffused zone of blazing out from the line of inoculation.
- Non-motile organisms grow along the line of inoculation. Degradation of the carbohydrate to acid is indicated by the pH indicator bromothymol blue which changes its color to yellow.
- Oxidative utilization takes place when the medium is exposed to air while fermentative utilization occurs under the exclusion of air. Generally, the test is performed in duplicate, of which one tube is covered with mineral oil to exclude oxygen and the second tube is uncovered; reactions of differential value can be observed.
- Fermentative organisms produce an acid reaction in both the covered and uncovered medium. Oxidative organisms produce an acid reaction in the uncovered medium and give slight growth without change in the covered medium.
- Organisms that are not classified either as oxidative or fermentative show no change in the covered medium and an alkaline reaction in the uncovered medium. The acidic reaction of oxidative organisms is more apparent at the surface of the medium that gradually spreads throughout the medium.
- If the oxidation reaction is weak or slow, an initial alkaline reaction at the surface of the uncovered tube may persist for several days and eventually convert to an acid reaction. The media can be incorporated with 2% serum or yeast extract (0.1%) to make the medium more nutritious.
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