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Principle
- Selenite cystine broth is composed of tryptone, lactose, disodium phosphate, L-cystine, and sodium hydrogen selenite. Tryptone provides nitrogen and amino acids. Lactose is a fermentable carbohydrate.
- Disodium phosphate provides phosphate for the growth and helps in the maintenance of the pH of the medium. L-cystine help in the enrichment of Salmonella species. Sodium selenite is a selective agent and inhibits the growth of gram-positive and many gram-negative organisms.
- Although sodium selenite is reduced by bacterial growth, and rise in pH takes place which leads to a decrease in the selectivity of selenite. Since lactose is added into the medium, due to lactose fermentation acid is produced and maintains neutral pH.
- The decrease in pH due to the reduction of selenite is compensated by the acid produced by lactose fermentation.
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