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Triple Sugar Iron Agar (TSI 3)
The purpose of this test is to determine whether organisms can ferment glucose, sucrose and/or lactose with or without production of gas. The ability of the organism to produce hydrogen sulphide from thiosulphate in an acid environment is also tested. Typically used to help differentiate between groups, genera and species among Enterobacteriaceae. Fermentation of glucose alone will show as a yellow colour in the butt of the medium, fermentation of sucrose and/or lactose will cause both butt and slant to be yellow.

Bacteria that attack glucose by oxidation or not at all are often obligate aerobes and will only grow on the slant of the agar and often cause an alkaline reaction there due to the utilisation of peptones.

Production of hydrogen sulphide leads to blackening. Results are given as slant/butt/gas production/hydrogen sulphide production.

1. Uninoculated medium.
2. Alkaline/neutral: Pseudomonas fluorescens, which attacks sugars oxidatively.
3. Alkaline/neutral: Alcaligenes, doesn't attack sugars.
4. Alkaline/neutral/hydrogen sulphide: Shewanella, does not attack sugars.
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© Department of Veterinary Disease Biology 2011
Faculty of Health and Medical Sciences - University of Copenhagen