Half Fraser Broth

Half Fraser Broth enrichment medium is intended for the isolation of Listeria spp. from food and environmental samples when used as the secondary enrichment medium according to ISO 11290-1 for Listeria isolation.

The medium contains ferric ammonium citrate and lithium chloride. All Listeria spp. hydrolyse aesculin to aesculetin. Aesculetin reacts with ferric ions which results in blackening. Another possible advantage to the addition of ferric ammonium citrate is that it has been shown that ferric ions enhance the growth of Listeria monocytogenes². Lithium chloride is included in the medium to inhibit the growth of enterococci which can also hydrolyse aesculin. Blackening of the medium is presumptive evidence of the presence of Listeria. Contrary to early indications, cultures which do not blacken cannot be assumed to be Listeria-free. Therefore, all Half Fraser Broth enrichment cultures should be sub-cultured to plating medium.

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