Selective Chromogenic Listeria Supplement Condalab 6040

This is a selective supplement for the isolation and confirmation of Listeria monocytogenes formulated according to ISO 11290-1 and 2:1996 Amd 2004. When completed with its supplements, the Agar Listeria Ottaviana & Agosti is a selective and differential medium for the detection of Listeria species and the presumptive identification of Listeria monocytogenes.

The selectivity is achieved by the high concentration of lithium chloride and the mixture of antimicrobics. The differential activitiy is due to the chromogenic substrate to detect the βglucosidase, enzyme that is present in all Listeria species. Specific identification is found by the L-α-phosphatidylinositol, which acts as a substrate for a phospholipase C present only in Listeria monocytogenes and some strains of Listeria ivanovii.

The combination of both substrates allows the differentiation L. monocytogenes, which produces colonies blue-green in colour and surrounded by an opaque zone, from the other Listeria species, which growth with blue-green colonies but without any halo. This differentiation is evident after incubate the plates for 24 ± 2 hours at 37 ºC. With highly contaminated samples, it is possible that some white in colour colonies do not have Listeria growth. In this case, it is recommended to perform an enrichment step previous to the plate inoculation.

Observations to make: Most Listeria ivanovii also produce an opaque halo around the colonies after 48 h of incubation. This presumptive evidence must be confirmed by performing the biochemical or serological identification tests (Rhamnose / Xylose sugar fermentation, hemolysis tests, CAMP test, etc.) or any test confirming the species without hesitation.