Determination of the Presence of Listeria monocytogenes in Milk and Dairy Products: IDF Collaborative Study

A collaborative study was conducted on the recovery of viable Listeria monocytogenes from milk and dairy products (Camembert cheese, Limburger cheese, skim milk powder, and ice cream). Test portions were homogenized with Listeria-selective liquid enrichment medium and cultured at 30°C for 48 h. The enrichment culture was then subcultured onto a solid isolation medium at 37°C for 48 h. Suspected Listeria colonies were identified by appropriate conventional morphological, physiological, and biochemical tests. Five kinds of dairy matrixes were spiked with L. monocytogenes at 2 levels: 12 and 120 colony forming units (cfu)/25 g. Each of the 18 collaborating laboratories analyzed 15 blind test portions from each matrix, comprising 5 replicates at each spiking level and 5 uninoculated controls, for a total of 1350 analyses. The specificity of the method was 100%; its sensitivity was 94–100% at the high spiking level and 89–98% at the low spiking level, except for Limburger cheese, which was only 68%. No specificity or sensitivity differences were observed between laboratories for all matrixes at the high spiking level and for all except Limburger cheese at the low spiking level. The calculated 50% detection limit for all products except Limburger cheese was 1.6 cfu/25 g; the 50% detection limit for Limburger cheese itself was 4.1 cfu/25 g. The method was adopted first action by AOAC INTERNATIONAL.