Listeria Enrichment HiVeg™ Medium Base (UVM)

Principle And Interpretation

Listeriosis is caused by Listeria monocytogenes, a short gram-positive non-sporulating rod. The bacilli are commonly found in soil and in the intestines of many animals including birds, fish, barnyard animals, dairy cattle and household pets. It is transmitted to humans by foods contaminated with faecal matter, as well as by the consumption of animal foods contaminated with the bacilli (1). Listeria Enrichment Medium Base is used for the selective cultivation and isolation of L. monocytogenes from clinical samples. The medium was originally formulated by Donnelly and Baigent (3). It was later modified by decreasing the nalidixic acid concentration in the selective supplements and subsequently increasing the acriflavin concentration (6). University of Vermont Modification Medium (UVM) used a two-step selective enrichment medium resulting in a higher isolation rate of L.monocytogenes from meat products within 3-4 days. This UVM Broth is recommended as a primary enrichment broth for recovery of heat injured Listeria (2). Listeria Enrichment HiVeg™ Medium Base (UVM) is prepared by using vegetable peptones in place of animal based peptones which make the media free of BSE/TSE risks. HiVeg™ hydrolysate, HiVeg™ peptone No. 3, HiVeg™ extract and yeast extract provide nitrogenous and carbonaceous compounds, long chain amino acids and other necessary nutrients while esculin offers differential properties to the medium. Nalidixic acid and acriflavin hydrochloride together with higher concentration of phosphate render the medium selective for Listeria. Gram-negative and gram-positive organisms are inhibited by nalidixic acid and acriflavin hydrochloride respectively. The two-step selective enrichment method developed (3) results in a higher detection rate of L.monocytogenes from specimens and has the added advantage of only taking 3-4 days. For primary isolation inoculate 25 gm or 25 ml specimen in 225 ml Listeria Enrichment Medium Base with added UVM Selective Supplement I (FD136). After 24 hours incubation, spread 0.2 ml of this medium on Listeria Selective HiVeg^TM Agar (MV567) plate. Simultaneously transfer 0.1 ml of Enrichment broth to 10 ml of fresh Listeria Enrichment Medium Base with added UVM Selective Supplement II (FD137). For secondary enrichment after 24 hours spread 0.2 ml of this medium on Listeria Selective HiVeg™ Agar (MV567) plate. Note: Broth cultures of Listeria are more dangerous than colonies on agar plates, so proper precautions should be taken while handling.