Mode of Action
The Oxford Agar formulation is based on Columbia Agar with the addition of lithium chloride, acriflavin, colistin sulfate, cefotetan, cycloheximide and fosfomycin. These ingredients suppress the growth of the common bacteria (e.g. Gram-negative bacteria and a greater part of Gram-positive bacteria).
Lithium chloride is one of the ingredients of Oxford Agar base, whereas the other substances derive from the Oxford Listeria Selective Supplement.
Listeria monocytogenes hydrolyses esculin to esculetin and forms a black complex with iron(III)ions. Therefore Listeria monocytogenes produces brown-green coloured colonies with a black halo.
Typical Composition (g/litre)
Peptone 23.0; starch 1.0; sodium chloride 5.0; agar-agar 13.0 (=Columbia agar); esculin 1.0; ammonium iron(III) citrate 0.5; lithium chloride 15.0.
Suspend 29.25 g in 500 ml of demin. water, autoclave (15 min at 121 °C). Dissolve the lyophilisate of 1 vial Oxford Listeria Selective Supplement (Code:ZA.002.022) by adding 5 ml of a 1:1 mixture of ethanol and sterile distilled water. Mix gently and add the contents to the culture medium cooled to 50 °C. Pour the medium into plates and leave to solidify. pH: 7.0 ± 0.2 at 25 °C.
The prepared agar (incl. supplement) is clear and bluish-brown.
Experimental Procedure and Evaluation
Inoculate by spreading the sample on the surface of the medium and incubate at 35 °C up to 48 h aerobically. Listeria monocytogenes grows as brown-green coloured colonies with a black halo (esculin splitting). Further biochemical tests should be carried out