Chromogenic media for the detection and/or enumeration of Listeria monocytogenes - results of trials performed by a working group of the International Organization for Standardization - ISO/TC 34/SC 9

The solid selective media PALCAM and Oxford agar originally described in the ISO (International Organization for Standardization) Standard 11290 part 1 and part 2 "Microbiology of food and animal feeding stuffs - Horizontal method for the detection and enumeration of Listeria monocytogenes", suffer from the disadvan-tage that L monocytogenes cannot be differentiated from non-pathogenic liste-rias.On the basis of the results of trials, organised by an ISO/TC 34/SC 9 working group with blood-based and chromogenic media, ISO members agreed that the ALOA medium and Rapid'L.mono agar performed significantly better than the other media tested.Moreover, the ready-to-use ALOA plates performed similarly to the medium prepared from the individual components. Finally Substitution of the expensive component phosphatidylinositol (Pl), Substrate for phospholipase C activity of L monocytogenes, by lecithin had no noticeable effect on the elective features of the ALOA medium, provided the lecithin contained a sufficiently high concentration of Pl.