Triple sugar iron agar
From IDWiki
- Contains three sugars (lactose, sucrose, and glucose) as well as iron, within a solid agar medium
- Peptone as a source of nitrogen
- Medium is originally reddish due to the phenol red indicator, which turns yellow in acidic conditions
- Tubes contain a well-oxygenated slant and a poorly-oxygenated butt which need to be interpreted separately
- Inoculated with a stab down towards the butt followed by streaking the slant
- After 18 to 24 hours at 35°C with a loose cap, the final colour depends on the metabolism of the four components
- Glucose alone fermented: butt turns yellow (acid), slant stays red (alkaline) (Alk/A)
- Lactose or sucrose fermented: both butt and slant turn yellow (acid) (A/A)
- No fermentation of glucose, lactose, or sucrose: both butt and slant stay red (Alk/Alk)
- Iron: if hydrogen sulfide (H2S) is produced, the iron will turn black
- Gas may be formed with active fermentation of either glucose or lactose/sucrose
Interpretation
- Blackening suggests H2S production
| Slant | Butt | Gas | H2S | Fermentation | Examples |
|---|---|---|---|---|---|
| Red | Red | No | No | — | Pseudomonas |
| Red | Yellow | No | No | G | Shigella, Serratia |
| Yellow | Yellow | Yes | No | G/L/S | Escherichia, Klebsiella, Enterobacter |
| Yellow | Yellow | Yes | Yes | G/L/S | |
| Red | Yellow | Yes | Yes | G | Salmonella, Proteus |
