Introduction
Tetrathionate broth is a selective enrichment liquid medium used for the isolation of Salmonella species (except S. Typhi).
It contains tetrathionate ions and bile salts, which inhibit the growth of commensal intestinal bacteria, while allowing enteric pathogens like Salmonella to multiply.
Media Type
Category: Selective enrichment liquid medium
Purpose: Promotes the growth of Salmonella spp. while suppressing coliforms and other intestinal bacteria.
Composition & pH of Tetrathionate Broth
Typical composition per liter:
| Component | Function |
|---|---|
| Peptone | Source of nitrogen, amino acids |
| Bile salts | Inhibit Gram-positive and commensal coliforms |
| Calcium carbonate | Neutralizes acidity |
| Sodium thiosulfate + Iodine (added fresh) | Generate tetrathionate ions → selective agent |
| Distilled water | Solvent |
Final pH: 8.4 (alkaline)
Sterilization
The basal medium (without iodine solution) is sterilized by autoclaving at 121°C for 15 minutes.Before use: Add fresh iodine–iodide solution to the cooled broth → releases tetrathionate ions, which provide selectivity.
Preparation of Tetrathionate Broth
Prepare basal medium with peptone, bile salts, and calcium carbonate.
Autoclave at 121°C for 15 minutes.
Allow to cool to ~50°C.
Add freshly prepared iodine–iodide solution to release tetrathionate ions.
Mix gently and dispense into sterile tubes or bottles.
Uses of Tetrathionate Broth
Selective enrichment of Salmonella spp. from clinical specimens (stool, urine, water, food).
Suppresses commensals such as E. coli, Proteus, Klebsiella.
Especially important in enteric fever and gastroenteritis diagnosis.
Commonly used in public health and food microbiology for detecting Salmonella contamination.
Limitations
Not suitable for the isolation of S. Typhi, as it is inhibited by tetrathionate.
Overgrowth by Proteus spp. may sometimes occur.
Needs fresh iodine addition before use, which adds extra preparation steps.
Conclusion
Tetrathionate broth is a valuable selective enrichment medium for the isolation of non-typhoidal Salmonella spp. from fecal and food samples. Its selective action via tetrathionate ions and bile salts makes it crucial in clinical and public health microbiology.
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