Selenite F Broth (Twin Pack) Medium 11 (In accordance with IP 2007)

Principle And Interpretation

Klett (1) first demonstrated the selective inhibitory effects of selenite and Guth (2) used it to isolate Salmonella Typhi. Leifson fully investigated selenite and formulated the media. The formulation corresponds to that of recommended by the Indian Pharmacopoeia (3) for detection of Salmonella in foodstuffs, pharmaceuticals and pathological materials. Enrichment media are routinely employed for detection of pathogens in faecal specimens as the pathogens are present in a very small number in the intestinal flora. Selenite F Broth is useful for detecting Salmonella in the nonacute stages of illness when organisms occur in the test sample in low numbers and for epidemiological studies to enhance the detection of low number of organisms from asymptomatic or convalescent patients (4).

Peptone provides nitrogenous substances. Lactose maintains the pH of medium. Selenite is reduced by bacterial growth producing alkalinity. This causes increase in pH which can reduce the toxicity of the selenite and results in overgrowth of other bacteria. The acid produced by bacteria by lactose fermentation counters the high pH and neutralizes the medium. Sodium phosphate maintains a stable pH and also minimizes the toxicity of selenite.

Enriched broth is subcultured on differential plating media such as Bismuth Sulphite Agar (MM027), Brilliant Green Agar (MM016), XLD Agar (MM031) etc. Do not incubate the broth longer than 24 hours as inhibitory effect of selenite decreases after 6 - 12 hours of incubation (5).