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Triple Sugar Iron Medium (TSI medium) - Composition and bacteria cultured

Triple sugar iron medium image reload

Triple Sugar Iron Medium (TSI medium)
This medium is prepared in a test tube, the upper portion is called slant and the lower portion is called butt.

1. Glucose,
2. Sucrose
3. Lactose
4. Ferrous sulphate
5. Tissue extracts (proteins)
6. Phenol red (indicator)

Test tube.

Red before reaction (alkaline)
yellow after reaction (acidic)
Black after hydrogen sulphide gas production due to the formation of FeS.

Solid medium

This medium is used to differentiate Salmonella and Shigella from other enteric gram-negative rods in stool culture.

Micro-organisms/bacteria are cultured.
Salmonella and shigella produce: 
a). Alkaline slant (red)
b). Acidic but with no gas (yellow without bubbles)
c) salmonella produces hydrogen sulphide gas but Shigella don't.

Other enteric Gram-negative rods produce:
a. Acidic slant (yellow)
b. Acidic butt with gas ( yellow with bubbles) e.g E. coli
Table: Reactions shown by various genera in TSI.

Slant Butt Gas H2s Representative Genera
Acid (yellow)  Acid (yellow)  yes (bubbles)  No  Escherichia, Enterobacter, Klebsiella
Alkaline (red) Acid No No Shigella, Serratia
Alkaline Acid Yes Yes (black Spots) Salmonella, Proteus
Alkaline Alkaline No No Pseudomonas


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