Mode of Action
Optimum growth conditions are created for Listeria due to the high nutrient content and the large buffer capacity. The growth of accompanying bacteria is largely inhibited by lithium chloride, nalidixic acid and acriflavine hydrochloride. The detection of the b-D-glucosidase activity of Listeria is possible by the addition of esculin and amonium iron(III) citrate. The glucose esculin is cleaved by b-D-glucosidase into esculetin and glucose. The esculetin then forms an olive-green to black complex with the iron(III) ions. Therefore, during the growth of Listeria in FRASER
broth, usually a blackening of the broth is observed. An improved enrichment of Listeria in comparison with the standard method can be attained using the two-step enrichment method with an initially halved concentration of nalidixic acid and acriflavine hydrochloride.
Typical Composition (g/litre)
Proteose peptone 5.0; peptone from casein 5.0; yeast extract 5.0; meat extract 5.0; sodium chloride 20.0; disodium hydrogen phosphate 9.6; potassium dihydrogen phosphate 1.35; esculin 1.0; lithium chloride 3.0.
Suspend 55.0 g in 1 litre demin. water and autoclave (15 min at 121 °C). To prepare half-concentraded FRASER broth, dissolve the contents of 1 vial amonium iron(III) citrate and 1 vial of selective supplement (Cat. No. 1.10399.0001 FRASER Supplement) in 1 ml of sterile distilled water each and add to the broth after it has cooled below 50 °C. FRASER broth is made by adding a further bottle of selective supplement to the halfconcentrated FRASER broth. The supplements are homogeneously distributed in the broth by carefully swirling.
pH: 7.2 ± 0.2 at 25 °C.
The prepared broth is clear to almost clear and yellowish-brown
1. Enrichment step The half-concentrated FRASER broth is inoculated with sample material and incubated at 30 °C for 24 ± 2 hours. From this culture, a selective growth medium such as OXFORD or PALCAM Agar is inoculated.
2. Enrichment step From the first enrichment step, 0.1 ml is inoculated on to 10 ml FRASER broth for two incubations of 48 ± 2 hours at 35 °C or 37°C. After each 24 hours period selective growth media such as
OXFORD and/or PALCAM agar are inoculated