Listeria Selective Broth Base, Granulated

Principle And Interpretation

Listeria monocytogenes is a short, gram-positive, non spore-forming rod shaped bacterium that appears coccoidal in older cultures. Listeria multiplies over a wide range of temperatures from 3°C to 45°C with optimum temperature range of 30°C to 37°C. L.monocytogenes has been isolated from numerous environmental sources such as silage, soil, decaying vegetation, sewage, damp earth, straw and faeces (1,2). Detection of L.monocytogenes in foods is not difficult. Low numbers of organisms are commonly isolated from raw milk, meat, vegetables, seafood and the food-processing environment. Enrichment procedures are used to isolate low numbers of L.monocytogenes. Injured L. monocytogenes are sublethally stressed as a result of exposure to heat, freezing or acidic conditions. Sublethally stressed L.monocytogenes require resuscitation in a non-selective medium at a temperature favouring repair of the sublethal injury. Listeria Selective Broth is formulated as per Lovett et al (3) for the selective enrichment of Listeria species from milk and milk products and other foods. Listeria Selective Broth is recommended by ISO Committee (4) with a slight modification in the supplement (FD063I).

Tryptone, soya peptone and yeast extract provide carbon and nitrogen compounds, long chain amino acids, vitamins essential for bacterial metabolism. Dextrose is the energy source. The medium is rendered selective by addition of selective supplement. Cycloheximide inhibits the growth of saprophytic fungi. Nalidixic acid inhibits growth of gram-negative organisms and acriflavin suppresses gram-positive microorganisms (5,6). Acriflavin is an acridinic derivative with bacteriostatic properties towards many gram-positive bacteria and a weak fungicidal activity. For enrichment, 25 grams or 25 ml sample is added to 225 ml medium in a stomacher bag. Homogenize the material if required. Incubation is carried out at 30°C for upto 7 days. Ajello et al (7) showed that incubation period of 7 days allows better recovery of environmentally stressed Listeria from milk and milk products. The enrichment broth is further subcultured on Listeria Selective Agar (M567) after 1, 2 and 7 days. Listeria monocytogenes is a highly pathogenic organism and therefore proper precautions should be taken while handling them.