Sodium Biselenite Bud

Principle And Interpretation

Selective inhibitory effects of selenite were first demonstrated by Klett (1). Guth (2) used it to isolate Salmonella Typhi. Leifson studied selenite and formulated a medium using selenite. Different formulation of Selenite Cystine Medium has been recommended by AOAC (5) for the detection of Salmonella in foodstuff, particularly egg products. It is also recommended by APHA (6, 7),USP (8), IP (14), BIS (4), ISO (3). Selenite Cystine Broth is useful for detecting Salmonella in the non-acute stages of illness when organisms occur in the faeces in low numbers and for epidemiological studies to enhance the detection of low numbers of organisms from asymptomatic or convalescent patients (9). Salmonella are also injured during various food processing procedures, including exposure to low temperatures, sub-marginal heat, drying, radiation, preservatives or sanitizers. Recovery of Salmonella involves pre- enrichment, selective enrichment and selective plating since Salmonella may be present in low numbers in food sample in a injured conditions. This medium is formulated to allow the proliferation of Salmonella while inhibiting the growth of competing non - Salmonella organisms.

Tryptone provides nitrogenous substances. Lactose/ dulcitol is the fermentable carbohydrate and maintains the pH in medium as selenite is reduced by bacterial growth and alkali is produced. An increase in pH lessens the toxicity of the selenite and results in overgrowth of other bacteria. The acid produced by bacteria due to lactose/dulcitol fermentation serves to maintain a neutral pH. Phosphate maintains a stable pH and also lessens the toxicity of selenite. L-cystine is the reducing agent, improving the recovery of Salmonella. Enriched broth is subcultured on solid medium. Do not incubate the broth longer than 24 hours as inhibitory effect of selenite reduces after 6 12 hours of incubation (10). Inoculate the sample into recommended pre-enrichment broth, and then transfer 1 ml of mixture to 10 ml of Fluid Selenite Cystine Medium and also to 10 ml Tetrathionate Broth (M032). Incubate and subsequently subculture on to Bismuth Sulphite Agar (M027), Xylose-Lysine-Deoxycholate Agar (M031), Hektoen Enteric Agar (M467) or MacConkey Agar (M081) as recommended by various organization.