Listeria monocytogenes hydrolyzes esculin to form esculetin and dextrose. Esculetin
reacts with ammonium ferric citrate and forms a brown-black complex seen as medium
blackening. Lithium chloride inhibits the growth of enterococci which can also hydrolyse
Inoculate 10ml of Fraser Broth with 0.1ml of the primary enrichment broth (Half Fraser) which has been incubated for 20 to 24 hours.
Incubate at 35 °C for 26 ± 2 hours, aerobically. Even if darkens or not, tubes should be subcultured on Oxford or PALCAM agar.
See also ISO 11290-1:1996 Microbiology of food and animal feeding stuffs - Horizontal method for the detection and enumeration of
Listeria monocytogenes - Part 1: Detection method, and ISO 11290-2:1998 Microbiology of food and animal feeding stuffs - Horizontal
method for the detection and enumeration of Listeria monocytogenes - Part 2: Enumeration method.
RESULTS & INTERPRETATION
Medium blackening means esculin hydrolysis and presumptive Listeria monocytogenes presence. Unchanged yellow color cannot
exclude Listeria and culture should be inoculated on plating media for absence confirmation.
Listeria monocytogenes ATCC 7644 - blackening.
Pseudomonas aeruginosa ATCC 27853 - yellow.
Peptic digest of animal tissue 5.0 g, yeast extract 5.0 g, casein enzymic hydrolysate
5.0 g, meat extract 5.0 g, sodium chloride 20.0 g, Na2HPO4 * 2H2O 12.0 g, KH2PO4
1.35 g, lithium chloride 3.0 g, esculin 1.0 g.
Distilled water ad 1000 ml, pH 7.2 ± 0.2.
FRASER SUPPLEMENT (for 500ml base medium)
Ferric ammonium citrate 500.0 mg, acriflavine hydrochloride 25.0 mg, nalidixic acid
HALF FRASER SUPPLEMENT (for 500ml base medium)
Ferric ammonium citrate 500 mg, acriflavine hydrochloride 12.5 mg, nalidixic acid
Sterilise by autoclaving at 121°C for 15 minutes. Cool to 50°C, and aseptically add the
corresponding selective supplement (Fraser / Half Fraser).
Lithium chloride is harmful. Avoid bodily contact and inhalation of vapours.
Fraser Broth Base with added supplement is recommended, as a primary as well as secondary enrichment medium, for the isolation
and enumeration of Listeria monocytogenes from food and animal feeds.
(c) Costin Stoica