McBride Listeria HiVeg™ Agar Base

Principle And Interpretation

The disease listeriosis is a frequent cause of abortions in cattle and sheep. In human, symptoms are manifested as septicemia, encephalitis and circulatory monocytosis (1). Listeria multiplies over a wide range of temperatures, from 3°C to 45°C, and over a pH range of 5.0 to 9.6. It also survives in food products with pH levels outside these parameters (2). Because of these properties, Listeria survives the various food processing techniques (3). McBride Listeria Agar (4), recommended by APHA (5) is used for isolating Listeria from clinical specimens and foodstuffs including raw milk (6). This medium helps in the detection of low numbers of L. monocytogenes present in food samples. McBride Listeria HiVeg™ Agar Base is prepared by using vegetable peptones in place of animal based peptones which make the media free of BSE/TSE risks.

HiVeg™ hydrolysate No. 1 and HiVeg™ extract in the medium supply nitrogen, carbon, sulphur and trace nutrients required for the growth of Listeria. Phenyl ethyl alcohol is bacteriostatic for gram-negative bacteria as it selectively inhibits DNA synthesis (7). Sodium chloride maintains the osmotic balance of the medium. Glycine inhibits certain gram-negative and gram-positive bacteria including Escherichia coli and Enterococcus faecalis, the common accompanying contaminants. Lithium chloride also has antibacterial activity. Further selectivity is achieved by the addition of McBride Listeria Supplement (FD070). The detection of L. monocytogenes is greatly improved by pre-enrichment in liquid media either by one step or two steps. In one step method (8), infected material is inoculated directly in Listeria Selective HiVeg™ Broth Base (MV889), while in two steps method (9), infected material is inoculated in Listeria Enrichment HiVeg™ Medium Base (UVM) (MV890A) and incubated at refrigeration temperature of 4°C for few weeks (cold enrichment), as the organism has the ability to grow in low temperature. It is then inoculated in Fraser Secondary Enrichment HiVeg™ Broth Base (MV1083), followed by plating onto selective agar such as McBride Listeria Agar. The presumptive Listeria colonies are selected under 45° transillumination. Listeria colonies are dense white to iridescent white appearing as crushed glass. Small colonies tend to be blue, while non-Listeria show yellowish orange colonies that are further purified. McBride Listeria Agar can be used as a plating medium with or without supplementation of blood