SELECTIVE AND DIFFERENTIAL MEDIUM FOR ISOLATION OF LISTERIA-MONOCYTOGENES FROM FOODS

Hemolytic ceftazidime lithium chloride agar (HCLA) has been developed as a selective and differential plating medium for the isolation of Listeria monocytogenes from fishery products. Selectivity is based upon lithium chloride, colistin methane sulfonate, and ceftazidime. When horse blood was incorporated in the agar overlay, L. monocytogenes colonies were easily distinguished by their characteristic blue-gray color surrounded by narrow zones of beta-hemolysis. Listeria innocua, a species commonly present in foods, does not produce hemolysis on the medium. Therefore, one or more colonies of L. monocytogenes were easily distinguished from large populations of L. innocua. When used in combination with Food and Drug Administration and U.S. Department of Agriculture enrichment methodology, HCLA was effective in inhibiting competitive organisms, differentiating colonies of L. monocytogenes by their beta-hemolysis, and shortening the incubation time at 35-degrees-C for presumptive identification to 17-24 h.