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Principle
- Fraser Broth Base is based on the formulation of Fraser and Sperber (1988) and is used for the detection of Listeria species in food products. It is composed of peptone, tryptone, yeast extract, meat extract, sodium chloride, sodium phosphate dibasic, potassium phosphate monobasic, esculin, and lithium chloride.
- The media can be fortified with nalidixic acid and acriflavine to increase the selectivity of media. Peptone, tryptone, meat extract, and yeast extract provide carbon, nitrogen sources, and essential growth factors required for the growth of Listeria.
- The high sodium chloride concentration of the medium inhibits the growth of enterococci. Sodium phosphate and potassium phosphate act as buffering agents. The Listeria species has the ability to hydrolyze esculin to esculetin.
- The esculetin reacts with ferric ammonium citrate (Fraser Selective supplement (RDM-FSS-02)), to form a black color complex (6-7 dihydroxycoumarin). Lithium chloride inhibits gram-negative bacilli. Selectivity is provided by nalidixic acid and acriflavine (Fraser Selective supplement (RDM-FSS-01)). The nalidixic acid inhibits gram-negative bacteria and acriflavine inhibits gram-positive bacteria.
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