Use of a novel medium, the Polymyxin Ceftazidime Oxford Medium, for isolation of Listeria monocytogenes from raw or non-pasteurized foods

Polymyxin Ceftazidime Oxford Medium (PCOM), a novel selective and differential plating medium for Listeria monocytogenes was compared with Modified Oxford Agar (MOX) for efficacy to isolate L. monocytogenes and other Listeria spp. naturally present in non-pasteurized Mexican-style cheese (n = 50), non-pasteurized fresh squeezed orange juice (n = 50), raw beef chunks (n = 36), and fresh cabbage (n = 125). Samples were collected from retail markets and farms in Mexico and tested following the US Department of Agriculture enrichment technique. Listeria spp. were isolated from 23.4% of analyzed samples, and from those, 75.0% corresponded to raw beef chunks, 38.0% to non-pasteurized Mexican-style cheese, and 30.0% to fresh squeezed orange juice. No Listeria spp. were isolated from fresh cabbage samples. L. monocytogenes was recovered from 15.3% of food samples analyzed. Non-pasteurized Mexican-style cheese showed the highest proportion of L. monocytogenes positive samples (36.0%), followed by orange juice (26.0%) and raw beef (25.0%). The frequency of isolation of Listeria spp. and L. monocytogenes was not different (P > 0.05) between PCOM and MOX. The advantages of using PCOM when comparing to MOX, include the easier way to identify Listeria species, the lower cost per plate and the availability of its ingredients for Latin-American countries.