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 Li steria monocytogenes from fishery products. Selectivity is based upon lithium chloride, colistin methane sulfonate, and ceftazidime. When ho rse blood was incorporated in the agar overlay, L. monocytogenes colon ies were easily distinguished by their characteristic blue-gray color surrounded by narrow zones of beta-hemolysis. Listeria innocua, a spec ies commonly present in foods, does not produce hemolysis on the mediu m. Therefore, one or more colonies of L. monocytogenes were easily dis tinguished from large populations of L. innocua. When used in combinat ion with Food and Drug Administration and U.S. Department of Agricultu re enrichment methodology, HCLA was effective in inhibiting competitiv e organisms, differentiating colonies of L. monocytogenes by their bet a-hemolysis, and shortening the incubation time at 35-degrees-C for pr esumptive identification to 17-24 h.