VLD 421: Veterinary Clinical Biochemistry and Laboratory Diagnosis-II (0+1)

TRIPLE SUGAR IRON AGAR TEST (TSI)

Principle

• Triple Sugar Iron (TSI) agar is used to find out whether the organisms can utilize glucose (0.1%), lactose (1%) and sucrose (1%) fermentatively and forms hydrogen sulphide with or without production of gas.
• Enterobacteriaceae can ferment all the three sugars and produce acid, which is indicated by yellow colour by the presence of phenol red indicator.
• Some other members can only ferment limited amount of glucose available in the media and subsequently will utilize peptones in the medium as they can not attack lactose and sucrose. The metabolites of peptones are alkaline and causes the slant to revert back to the original red colour.
• It can also indicate the production of hydrogen sulphide. The production of H₂S is detected by indicators such as ferrous sulfate, ferric citrate, lead acetate etc by forming an insoluble, heavy metal sulfide black precipitate.

Methodology

• Inoculate the TSI slant with the test organism by using a inoculation wire.
• Incubate at 37^ºC for 24 to 48 hours.

Result

• The results can be interpreted as follows:

Triple sugar iron agar test

1 - Yellow slant and yellow butt with air bubbles	Acid from lactose or sucrose or both with gas production
2 - Red slant and yellow butt	Acid from glucose only. Lactose and sucrose are not utilized.
3 - Blackening of the butt	Hydrogen sulphide production.
4 - Red slant and red butt	No fermentation