-
Half-Fraser broth is a selective enrichment broth for the identification and isolation of Listeria spp., primarily from food and environmental samples. The antibiotics, nalidixic acid and acriflavine, are already included in the formulation so only ferric ammonium citrate (LS5004) need be added to make the complete Half-Fraser broth. The peptones provide carbon, nitrogen and vitamins, sodium chloride provides osmotic balance and the phosphate buffer system maintains pH. Lithium chloride inhibits enterococci and the antibiotics make the medium highly selective. Listeria spp. hydrolyse aesculin to aesculetin which forms a confirmatory dark brown or black complex with Fe3+ ions. -
Violet Red Bile Agar is a medium for the enumeration of coliform organisms in food and dairy products and conforms to American Public Health Association (APHA). Yeast extract and enzymatic digest of gelatin provides the required carbon, nitrogen and vitamins. Sodium chloride maintains the osmotic balance. The medium is made selective by the inclusion of bile salts and crystal violet to inhibit Gram-positive and non-enteric organisms. Lactose is a fermentable carbohydrate and neutral red is a pH indicator. Lactose fermenters produce red/purple colonies often surrounded by a halo of the same colour. Non lactose fermenters produce pale colonies. Selectivity can be increased by incubation at 42-44ºC.
