Listeria Chromogenic Selective Supplement
Description:
Completed with all its supplements the Agar Listeria Ottaviani & 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 activity is due to the chromogenic substrate to detect the βglucosidase enzyme that is present in all Listeria species.
The specific identification is obtained by the L-α-phosphatidylinositol, that acts as substratre 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 grow in produces colonies blue-green in colour and surrounded by an opaque zone, from the other Listeria species, which blue-green colonies but without any halo. This differentiation is evident after incubating the plates for 24 ± 2 hours at 37 ºC.
Sometimes, especially with highly contaminated samples, it is possible that some colonies, white in colour, are not Listeria growth. In this case an enrichment step is recommended prior to plate inoculation.
Observations:
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.
Technique:
Add 1 botlle supplement Ottaviani & Agosti (L-alpha-phosphatidylinositol) and 1 vial supplemet Ottaviani & Agosti for complete 500 ml medium.
Homogenize by mixing and distribute in Petri dishes. The solidified cool medium appears homogeneously turbid.
There are many standardised methodologies (ISO, FDA-BAM, AOAC, AFNOR, etc.). The technician must follow the protocol validated in his laboratory.