Lauryl Sulphate HiVeg™ Broth (Lauryl Tryptose HiVeg™ Broth)

Principle And Interpretation

Lauryl Tryptose HiVeg® Broth is prepared by replacing tryptose with HiVeg® hydrolysate No.1, making the medium free of BSE/TSE risks. This can be used as an alternative for Lauryl Tryptose Broth which was formulated by Mallmann and Darby (1) and is recommended by APHA and ISO committee (2) for the presumptive detection of coliforms in water, effluent or sewage by MPN test and for the detection of coliforms in foods (3). Coliforms are considered to be members of Enterobacteriaceae, which grow in presence of bile salts and produce acid and gas from lactose within 48 hours at 37°C. They generally show ẞ-galactosidase activity (4). Cowls (5) demonstrated that inclusion of sodium lauryl sulphate (SLS) makes the medium selective for coliform bacteria (1). Aerobic spore bearers are completely inhibited. This media has also been reported to provide a higher colon index than the confirmatory standard methods media and the gas production in this not only acts as a presumptive test, but also as a confirmatory test for the presence of coliforms, in the routine testing of water. Aerobic spore-bearers are completely inhibited in this medium.

HiVeg® hydrolysate No.1 provides essential growth substances, such as nitrogen and carbon compounds, sulphate and trace ingredients. The potassium phosphates provide buffering system, while sodium chloride maintains osmotic equilibrium. For inoculum of 1 ml or less, use single strength medium. For inocula of 10 ml or more, double strength or proportionate medium should be prepared. After inoculation, incubate the tubes at 35-37°C for 24 to 48 hours. For every tube showing fermentation (primary fermentation), inoculate two tubes of Lauryl Tryptose HiVeg® Broth from the tube showing primary fermentation and incubate these tubes at 35-37°C and 44°C respectively. If there is fermentation in the tube incubated at 44°C after 8 to 24 hours, perform indole test by adding Kovac's reagent. A positive indole test in a broth tube showing gas production at 44°C indicates the presence of Escherichia coli. If no fermentation occurs in the tube incubated at 35-37°C after 24 hours, the primary fermentation is assumed to be due to organisms other than coliforms. Broth becomes cloudy or forms precipitate if stored at 2 - 8°C, but it should get cleared at room temperature.