|Listeria monocitogenes colonies on Oxford Agar
See 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
May also be used for direct inoculation
Listeria monocytogenes ATCC 7644 - good growth, brown-black colonies with black
Enterococcus faecalis ATCC 29212 - inhibited;
Bacillus subtilis ATCC 6633 - inhibited;
Escherichia coli ATCC 25922 - inhibited.
Columbia Blood Agar Base 39.0 g, Aesculin 1.0 g, Ferric ammonium citrate 0.5 g,
Lithium chloride 15.0 g, Distilled water ad 1000 ml, pH 7.2 ± 0.2.
SELECTIVE SUPPLEMENT (for 500ml base medium)
Cycloheximide 200.0 mg, Colistin sulphate 10.0 mg, Acriflavine 2.5 mg, Cefotetan
1.0mg, Fosfomycin 5.0 mg.
MODIFIED SELECTIVE SUPPLEMENT (OXFORD)
Same formulation except Amphotericin B 5 mg replaces Cycloheximide 200 mg.
Sterilise by autoclaving at 121°C for 15 minutes. Cool to 50°C, and aseptically add the
Lithium chloride is harmful. Avoid bodily contact and inhalation of vapours.
Variants of medium replace Columbia Blood Agar Base & yeast extract with:
Peptic digest of animal tissue 23.0 g, Starch 1.0 g, Sodium chloride 5.0 g.
Medium contains ferric citrate. L. monocytogenes hydrolyzes esculin to form
esculetin and dextrose. Esculetin reacts with ammonium ferric citrate and forms a
brown-black complex seen as a black halo around colonies. Gram-negative bacteria
are completely inhibited. Other Gram-positive are inhibited or grow poorly.
Listeria Selective Medium (Oxford Formulation) is based on the formulation described by Curtis et al. and is recommended for the
detection of Listeria monocytogenes from clinical and food specimens.
(c) Costin Stoica