Triple Sugar Iron (TSI) Agar Test: Introduction, Composition, Principle, Test Requirements, Procedure, Result- Interpretation, Keynotes, and TSI Test Footages
Table of Contents
The triple Sugar Iron (TSI) Agar test is a combo test and is useful for the identification of mainly gram-negative bacteria, on the basis of the following tests-
Triple Sugar Iron Agar (TSI) (Oxoid, England)
| Ingredients | gm/liter |
| Lab-lemco powder | 3.0 |
| Yeast Extract | 3.0 |
| Peptone | 20.0 |
| Lactose | 10.0 |
| Sucrose | 10.0 |
| Glucose | 1.0 |
| Ferric Citrate | 0.3 |
| Sodium Chloride | 5.0 |
| Sodium Thiosulphate | 0.3 |
| Phenol Red | 0.024 |
| Agar | 12.0 |
| Final pH (at 25ºC) | 7.4±0.2 |
The Triple Sugar Iron (TSI) Agar contains casein and meat peptones, phenol red as the pH indicator, 0.1% glucose, 1% lactose, and 1% sucrose for fermentation. Ferric or ferrous ions and sodium Thiosulphate are present to detect hydrogen Sulphide production. Organisms that are non-lactose fermenting initially produce a yellow slant due to the production of acid from glucose. The small amount of glucose is rapidly depleted. Oxidative metabolism continues in the slant after the low concentration of glucose has been depleted, producing an alkaline pH from the aerobic breakdown of peptone; the slant turns red. There is no oxygen penetration into the butt and no oxidative metabolism; the butt remains acid and yellow.
Thus, a non-lactose fermenting organism yields an alkaline (K) slant over an acid (A) butt (K/A); red slant; yellow butt). Lactose fermenting and/or sucrose-fermenting bacteria continue to produce a large amount of acid in the slant and butt so the reaction in both remains acid (A/A; yellow slant; yellow butt). If the slant and butt remain neutral, the organism is not capable of fermenting glucose or other sugars (K/K; red slant; red butt). Gas production from sugar fermentation is indicated by bubbles, the fracturing of the medium, or displacement of the medium. Hydrogen Sulfide is produced by the action of the bacteria with sodium Thiosulphate. This is detected by the reduction of ferric ions to produce a black precipitate.
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