Selenite F Broth is the medium used for the selective enrichment of Salmonella spp from both clinical and food samples. It is a buffered Lactose Peptone Broth to which Sodium Biselenite is added as the selective agent.
Selenite-F Broth was devised by Leifson, who demonstrated that selenite was inhibitory for coliforms and certain other microbial species, such as fecal streptococci, present in fecal specimens, and thus, was beneficial in the recovery of Salmonella species. He found that the inhibited strains would eventually breakthrough, but if subcultures were made from the enrichment broth after 8–12 h incubation, the isolation of Salmonella was possible without overwhelming growth of many members of the intestinal flora.
F in Selenite F Broth is Faeces, because it is mostly used for the isolation of Salmonella and Shigella from faecal specimens.
Composition of Selenite F Broth
|Casein enzymic hydrolysate||5 gm/L|
|Sodium phosphate||10 gm/L|
|Sodium hydrogen selenite (NaHSeO3)||4 gm/L|
pH 7.1 ± 0.2 @ 25°C
Principle of Selenite F Broth
Casein enzymic hydrolysate provides nitrogenous substances, carbon compounds and vitamin sources. Lactose is a carbohydrate source which maintains the pH of medium. 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 fermentation serves to maintain a neutral pH. Sodium phosphate maintains a stable pH and also lessens the toxicity of selenite. Sodium selenite inhibits many species of gram-positive and gram-negative bacteria including enterococci and coliforms. Do not incubate the broth longer than 24 hours as inhibitory effect of selenite decreases after 6 – 12 hours of incubation.
Uses of Selenite F Broth
- Selenite F Broth is used as a selective enrichment for the cultivation of Salmonella spp.
- It is also used as an enrichment medium for the isolation of some species of Shigella.
- The broth is also recommended for the transport of strains of Vibrio cholerae, because these organisms can survive 2 to 5 days in Sodium Selenite Broth.
Preparation of Selenite F Broth
- Add 4.0 gm sodium selenite powder to distilled/deionized water.
- Add remaining 19.0 gm powder to above solution and bring volume to 1.0 liter.
- Gently heat and bring to boiling.
- Dispense into sterile test tubes, at least of a depth of 5 cm.
- Sterilize in boiling water bath or at 0 psi pressure at 100°C for 10 minutes.
- DO NOT AUTOCLAVE.
- Cool to room temperature prior to use.
Result Interpretation on Selenite F Broth
After incubation, record growth of organisms, indicated by turbidity in the medium.
Positive Result: Colorless, Good Growth.
Examples: Salmonella Typhimurium, Shigella sonnei, Salmonella enteritidis, etc.
Negative Result: pink with bile precipitate, Inhibited or no growth.
Examples: Escherichia coli, Enterococcus faecalis, etc.
Limitations of Selenite F Broth
- Sodium hydrogen selenite (Sodium biselenite) is very toxic, corrosive agent and causes teratogenicity. Handle with great care. If there is contact with skin, wash immediately with lot of water.
- Enrichment broths should not be used as the sole isolation medium. They are to be used in conjunction with selective and non-selective plating media to increase the probability of isolating pathogens, especially when they may be present in small numbers.
- Due to nutritional variation, some strains may be encountered that grow poorly or fail to grow on this medium.
Quality Control on Selenite F Broth
Positive control: Salmonella Typhimurium ATCC® 14028. Good Growth.
Negative control: Escherichia coli ATCC® 25922. Inhibited or no growth.
- E&O Laboratories Ltd- Selenite F Broth (BM0360).
- Sisco Research Laboratories Pvt. Ltd.- Selenite F Broth, Base w|o Selenite (83179).
- HiMedia Laboratories Pvt. Ltd.- Selenite Broth (Selenite F Broth) (Twin Pack) (M052).
- Becton, Dickinson and Company, BBL- Selenite-F Broth (L007497).
- Scharlab- Selenite Broth Base (02-598).
- Acumedia Manufacturers, Inc.- SELENITE BROTH (7155).
- Conda Lab- Sodium Selenite Broth (1222).
- Titan Media- SELENITE F BROTH (TMV 389).
- OXOID PRODUCT SPECIFICATION- SELENITE F BROTH (EB0213E).
- Mast Diagnostics- Selenite F Broth