Listeria Chromogenic Agar Base according to Ottaviani and Agosti ISO Condalab 1345

Listeria Chromogenic Agar Base According to Ottaviani and Agosti is used for the presumptive isolation and identification of Listeria monocytogenes and Lesteria spp. in food and clinical samples. It is used for confirmation after using Listeria Enrichment Broth Base Fraser. This medium is also recommended by ISO 11290-1 for the detection and enumeration for Listeria monocytogenes.

The enzymatic digest of animal tissues and of casein provides nitrogen, vitamins, minerals, and amino acids that are essential for growth. Yeast extract is the source of vitamins, and sodium chloride that gives essential electrolytes for transport and osmotic balance.Sodium pyruvate is a source of energy for bacterial metabolism and aids in the resuscitation of stressed organisms. Glucose is the fermentable carbohydrate providing carbon and energy. Magnesium glycerophosphate is a buffering compound. Magnesium sulphate is a magnesium ion required for a large variety of enzymatic reactions, including DNA replication.

The differential activity of the medium is due to two factors. Lithium chloride in the base
medium and supplementary antimicrobial compounds Ceftazidime, Polymyxin, Nalidixic acid and Cycloheximide provide the medium’s selectivity. Bacteriological agar is the solidifying agent. The presence of the chromogenic component X-glucoside, a substrate for the detection of the enzyme ß-glucosidase, is common to all Listeria species
giving the colonies their blue colour. All other organisms that possess this enzyme, such as Enterococci, are inhibited by the selective agents within the medium and by the selective supplement. The differential activity is also obtained by lipase C substrate, upon which the specific enzyme for L. monocytogenes acts.

The lipase is responsible for the opaque white halo which surrounds L. monocytogenes.
The combination of both substrates allows us to differentiate the colonies of Listeria monocytogenes from the rest of Listeria spp. since, although all are
blue in colour, L. monocytogenes present an opaque white halo surrounding them.
It has been observed that some strains of Listeria ivanovii, mostly pathogenic to animals although some have caused infections in humans, also possess
lipase activity.