Difco™ Selenite Cystine Broth

Selenite Cystine Broth is used as a selective enrichment medium for the isolation of Salmonella from feces, foods, water and other materials of sanitary importance.

Summary and Explanation
Leifson found that selenite inhibited fecal streptococci and coliforms during the first 8-12 hours of incubation, thereby permitting salmonellae to replicate without overwhelming interference from other members of the intestinal flora. North and Bartram modified Leifson’s Selenite-F Enrichment broth by adding cystine, which stimulated growth of Salmonella.

User Quality Control
Identity Specifications
Difco™ Selenite Cystine Broth
Dehydrated Appearance: Off-white, free-flowing, homogeneous.
Solution: 2.3% solution, soluble in purified water upon boiling. Solution is very light amber, may have a trace tint of pink orange, clear to very slightly opalescent, may have a slight precipitate.
Prepared Appearance: Light amber, clear to slightly opalescent, may have a slight precipitate.
Reaction of 2.3%
Solution at 25°C: pH 7.0 ± 0.2

Cultural Response
Difco™ Selenite Cystine Broth
Prepare the medium per label directions. Inoculate and incubate at 35 ± 2°C for 24 ± 2 hours. After incubation, subculture onto Mac-Conkey Agar plates and incubate plated media at 35 ± 2°C for 18-24 hours.

The cystine-containing formulation is recommended by the Food and Drug Administration, AOAC International and American Public Health Association for detecting Salmonella in foods and waters.
Selenite Cystine Broth and similar enrichment media are useful for detecting Salmonella in the nonacute stages of illness when the organisms occur in the feces in low numbers and for epidemiological studies to enhance the detection of low numbers of organisms from asymptomatic or convalescent patients.

Principles of the Procedure
Peptone provides amino acids and other nitrogenous substances.
Lactose provides a source of energy, and sodium phosphate buffers the medium to maintain the pH. Sodium selenite inhibits gram-positive bacteria and suppresses the growth of most gramnegative enterics other than Salmonella. L-cystine is incorporated to improve the recovery of Salmonella.

Formula
Difco™ Selenite Cystine Broth
Approximate Formula* Per Liter
Pancreatic Digest of Casein : 5.0 g
Lactose : 4.0 g
Sodium Phosphate : 10.0 g
Sodium Selenite : 4.0 g
L-Cystine : 0.01 g
*Adjusted and/or supplemented as required to meet performance criteria.