Introduction
Wilson & Blair’s Medium is a selective differential medium primarily used for the isolation and identification of Salmonella typhi from stool, urine, or food samples. It contains bismuth sulfite which inhibits most intestinal bacteria and allows the growth of S. typhi. The black metallic sheen produced by S. typhi colonies makes the medium highly useful in diagnostic microbiology.
Media Type
Category: Selective and Differential Agar medium
Purpose: Isolation and differentiation of Salmonella typhi (causative agent of typhoid fever).
Selectivity: Bismuth sulfite, brilliant green, and bile salts inhibit coliforms and Gram-positive bacteria.
Differentiation: S. typhi reduces sulfite → produces black colonies with a metallic sheen.
Composition & pH of Wilson & Blair’s Medium
Per liter of medium:
| Component | Function |
|---|---|
| Peptone (10 g) | Source of nitrogen and growth factors |
| Dextrose (5 g) | Fermentable carbohydrate |
| Beef extract (5 g) | Provides vitamins and growth factors |
| Bismuth sulfite (8 g) | Selective agent; inhibits coliforms |
| Ferrous sulfate (0.3 g) | Detects hydrogen sulfide production |
| Brilliant green (0.01 g) | Selective agent; inhibits Gram-positive and many Gram-negative bacteria |
| Agar (15 g) | Solidifying agent |
| Distilled water (1000 mL) | Solvent |
Final pH: 7.7 ± 0.2
Principle of Wilson & Blair’s Medium
Selective action:
Bismuth sulfite, brilliant green, and bile salts suppress coliforms and most Gram-positive organisms.
Differential action:
S. typhi reduces sulfite → produces black metallic sheen colonies due to H₂S production reacting with ferric/ferrous salts.
Coliforms (e.g., E. coli) are inhibited or produce pale colonies
Sterilization
The medium is heat-sensitive and should not be autoclaved.Sterilize by steaming at 100°C for 30 minutes on three consecutive days (Tyndallization).
Preparation of Wilson & Blair’s Medium
Suspend 63 g of dehydrated medium in 1 liter of distilled water.
Heat gently with constant stirring until dissolved.
Do not boil or autoclave.
Dispense into sterile Petri plates or tubes.
Sterilize by tyndallization (not autoclaving).
Cool and store at 2–8°C.
Colony Characteristics on Wilson & Blair’s Medium
Salmonella typhi: Black metallic sheen colonies.
Salmonella paratyphi A & B: Black colonies, sometimes less intense than S. typhi.
Coliforms (e.g., E. coli): Inhibited or produce small, pale colonies.
Uses of Wilson & Blair’s Medium
Selective isolation of Salmonella typhi from stool, urine, blood cultures, and food samples.
Differentiation between S. typhi and coliform organisms.
Used in public health laboratories for diagnosis of typhoid fever.
Applied in water and food microbiology to check for typhoid carriers.
Limitations
Heat-sensitive medium → cannot be autoclaved.
Some Salmonella spp. may not grow well.
Does not differentiate S. typhi from all other Salmonella without biochemical tests.
Conclusion
Wilson & Blair’s Medium is a highly selective medium for the isolation of Salmonella typhi. Its composition with bismuth sulfite and brilliant green makes it effective in suppressing coliforms while allowing S. typhi to produce characteristic black metallic sheen colonies.
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