Introduction
Selenite F Broth is a selective enrichment liquid medium primarily used for the isolation of Salmonella species, especially S. Typhi from feces, urine, sewage, and food samples.
It contains sodium selenite, which inhibits the growth of commensal intestinal bacteria while permitting Salmonella (and some Shigella) to multiply.
Media Type
Category: Selective enrichment broth
Purpose: Recovery of Salmonella and Shigella spp. from samples with heavy normal flora.
Composition & pH of Selenite F Broth
Typical composition per liter:
| Component | Function |
|---|---|
| Peptone | Provides nitrogen, amino acids, growth factors |
| Lactose | Maintains pH and aids selective action |
| Sodium phosphate | Buffer to maintain pH |
| Sodium selenite | Selective agent → inhibits coliforms & Gram-positives |
| Distilled water | Solvent |
Final pH: 7.0 ± 0.2 (at 25°C)
Sterilization
Do not autoclave (sodium selenite is heat-sensitive and toxic at high concentrations).
Instead: Heat medium to just dissolve ingredients, then dispense into bottles/tubes.
Store in the dark at 2–8°C (selenite is unstable in light).
Preparation of Selenite F Broth
Dissolve peptone, lactose, sodium phosphate, and sodium selenite in distilled water.
Heat gently with frequent stirring until completely dissolved.
Do not overheat or autoclave.
Dispense 10 mL volumes into sterile tubes or bottles.
Store at 2–8°C, protected from light.
Uses of Selenite F Broth
Enrichment medium for Salmonella spp., especially Salmonella Typhi in suspected enteric fever cases.
Also supports the growth of some Shigella spp.
Used in stool culture to isolate Salmonella in early infection stages when bacterial numbers are low.
Applied in food microbiology for detecting Salmonella contamination.
Limitations
Prolonged incubation (>24 hours) may kill Salmonella → should be subcultured within 6–12 hours.
Does not support the growth of all Shigella strains.
Highly toxic (sodium selenite) → must be handled with care.
Cannot be sterilized by autoclaving (risk of breakdown and loss of selectivity).
Conclusion
Selenite F Broth is an essential selective enrichment medium in microbiology for the isolation of Salmonella spp., particularly S. Typhi. Its proper preparation, short incubation, and timely subculture are critical for accurate results
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