New chromogenic plating media for detection and enumeration of pathogenic Listeria spp.—an overview

In recent years a number of selective chromogenic plating media for pathogenic Listeria spp. have been developed and marketed. Their advantages are direct detection and enumeration of pathogenic Listeria spp. utilizing cleavage of substrates by the virulence factor phosphatidylinositol-phospholipase C (PI-PLC) and, to a lesser extent, by phosphatidylcholin-phospholipase C (PC-PLC). There are two groups of such media: the first utilizes cleavage by PI-PLC of L-α-phosphatidyl-inositol, forming a white precipitation zone around the colony, combined with the chromogenic substrate 5-bromo-4-chloro-3-indoxyl-β-d-glucopyranoside for detection of β-d-glucosidase, which occurs in all Listeria spp. All Listeria spp. produce turquoise colonies on these media which include ALOA™, CHROMagar™ Listeria , BBL™ CHROMagar™ Listeria, and OCLA. The second group of media utilizes 5-bromo-4-chloro-3-indoxyl-myoinositol-1-phosphate, forming blue-turquoise colonies of pathogenic Listeria spp. and white colonies of non-pathogenic Listeria spp.. BCM™ Listeria monocytogenes plating medium, RapidʹL.mono and LIMONO-Ident-Agar belong to this group. Selective chromogenic L. monocytogenes plating media offer the attraction of rapid economic detection and enumeration of pathogenic Listeria spp. within 24 or 48 h of incubation at 36±1 °C. This overview summarises the characteristics of these chromogenic plating media, reviews important evaluations, and focuses on replacement of conventional by these chromogenic plating media, particularly for applications in the food industry.