Listeria Half-Fraser Broth Base ISO
Principles and uses
Listeria Half-Fraser Broth Base is a modification of Listeria Fraser Broth Base in which the nalidixic acid and acriflavine concentrations have been reduced to 10 mg/L and 12.5 mg/L respectively. The antibiotics are already included in the formula so it is only necessary to add the Ferric Ammonium Citrate Supplement.
Listeria spp. may be present in small numbers and are often accompanied by considerably larger numbers of other microorganisms, therefore selective enrichment is necessary.
Listeria Half-Fraser broth is used in this selective enrichment and enumeration of Listeria monocytogenes and other Listeria species in all food types, including milk and dairy products, and environmental samples. This formula adheres to ISO 11290. Enzymatic digest of casein, enzymatic digest of animal tissues and meat extract provide nitrogen, vitamins, minerals and amino acids essential for growth. Yeast extract is the source of vitamins, particularly of the B-group. Potassium phosphates act as a buffer system. All Listeria species hydrolyze esculin, which reacts with ferric ions producing a blackening of the medium. The addition of ferric ammonium citrate improves the growth of Listeria monocytogenes. Lithium chloride inhibits the growth of enterococci that can hydrolyze the esculin.
Preparation
Suspend 28,7 grams of the medium in 500 ml. of distilled water. Mix well and dissolve by heating with frequent agitation. Boil for one minute until complete dissolution. Sterilize in autoclave at 121 ºC for 15 minutes. Cool to 45-50 ºC and aseptically add one vial of Ferric Ammonium Citrate Supplement . Homogenize gently and dispense into sterile containers.