Answer :
Lactose is not fermented, but a little quantity of glucose is; this causes the oxygen-deficient butt to appear yellow (keep in mind that more media means more glucose), whereas the acid on the slant is oxidized by the organism into carbon dioxide and water, causing the slant to appear red (alkaline or neutral pH).
Triple sugar iron test: what is it?
When you see the name of this test, Triple Sugar Iron Agar, you must keep in mind that it is a test that contains iron as well as three sugars (Lactose, Sucrose, and Glucose), and it also contains agar as a solidifying agent (TSI is a semi-solid media having slant and butt).
Lactose, Sucrose, and Glucose are present in Triple Sugar Iron Agar (TSI) in the proportions of 10:10:1 (i.e., 10 parts Lactose (1%), 10 parts Sucrose (1%) and 1 parts Glucose (0.1%)).
Triple Sugar Iron Agar Test Interpretation
1) Lactose (or sucrose) fermentation results in the production of a significant amount of acid, which causes the phenol red indicator to turn yellow both in the butt and in the slant. Gases produced by some organisms cause bubbles and fissures to appear on the medium.
2) The oxygen-deficient butt will be yellow if lactose is not fermented but a tiny amount of glucose is (remember that butt has more glucose than slant, thus more media means more glucose), but on the slant the acid The organism will oxidize the (less acid as medium in slant is very little) to carbon dioxide and water, turning the slant red (alkaline or neutral pH).
3) The butt and the slant will both be crimson if neither lactose/sucrose nor glucose is fermented. The formation of ammonia from the oxidative deamination of amino acids might cause the slant to turn a deeper red-purple (more alkaline) (remember peoptone is a major constituent of TSI Agar).
4) The presence of ferrous sulfide, which is black, indicates the presence of H2S.
- Red/red: Nonfermentable glucose, lactose, and sucrose
- Red/yellow: Only fermentation of glucose
- Yellow/yellow: fermentor for glucose, lactose, and/or sucrose
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