Evaluation of selective direct plating media for their suitability to recover uninjured, heat-injured, and freeze-injured Listeria monocytogenes from foods

Abstract

Six direct plating media were evaluated for their suitability to recover uninjured, heat-injured, and freeze-injured cells of four strains of Listeria monocytogenes from four foods. Cells were inoculated into foods to achieve ca. 10(2) to 10(3), 10(4) to 10(5), or 10(5) to 10(6) viable cells per ml or g (low, medium, and high populations, respectively). No appreciable differences in recovery of the four test strains within a treatment were observed. Generally, recovery on all test media was similar and not markedly affected by freeze treatment. Modified Despierres agar and modified McBride Listeria agar yielded poorer recovery of heat-injured cells than did McBride Listeria agar and gum base-nalidixic acid-tryptone soya agar. Overall, gum base-nalidixic acid-tryptone soya agar was best for recovering L. monocytogenes from pasteurized milk and chocolate ice cream mix. Enumeration was complicated by the growth of background microflora present in Brie cheese and cabbage, especially at the low inoculum. Dominguez Rodriguez isolation agar was superior for recovering L. monocytogenes from Brie cheese, whereas modified Despierres agar was best for recovering the organism from cabbage. Direct plating procedures can successfully be utilized for recovering healthy and injured L. monocytogenes from foods containing low populations of background microflora.