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Triple Sugar Iron Test

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The Triple Sugar Iron (TSI) Test 

To determine the ability of an organism to ferment glucose, lactose, and sucrose, and their ability to produce hydrogen sulfide.
Principle
The Triple Sugar Iron (TSI) test is a microbiological test named for its ability to test a microorganism’s ability to ferment sugars and to produce hydrogen sulfide. An agar slant of a special medium with multiple sugars constituting a pH-sensitive dye (phenol red), 1% lactose, 1% sucrose, 0.1% glucose, as well as sodium thiosulfate and ferrous sulfate or ferrous ammonium sulfate is used for carrying out the test. 

. Due to the building of acid during fermentation, the pH falls. The acid base indicator Phenol red. is incorporated for detecting carbohydrate fermentation that is indicated by the change in color of the carbohydrate medium from orange red to yellow in the presence of acids.  In case of oxidative decarboxylation of peptone, alkaline products are built and the pH rises. This is indicated by the change in colour of the medium from orange red to deep red. Sodium thiosulfate and ferrous ammonium sulfate present in the medium detects the production of hydrogen sulfide and is indicated by the black color in the butt of the tube.
Composition of Triple Sugar Iron Test

Final pH 7.3 +/- 0.2 at 25ºC.
Method

1.    With a straight inoculation needle, touch the top of a well-isolated colony.
2.    Inoculate TSI by first stabbing through the center of the medium to the bottom of the tube and then streaking the surface of the agar slant.
3.    Leave the cap on loosely and incubate the tube at 35°-37°C in ambient air for 18 to 24 hours.
4.    Examine the reaction of medium.
Expected Results
  • An alkaline/acid (red slant/yellow butt) reaction: It is indicative of dextrose(Glucose) fermentation only.
  • An acid/acid (yellow slant/yellow butt) reaction: It indicates the fermentation of dextrose, lactose and/or sucrose.
  • An alkaline/alkaline (red slant, red butt) reaction: Absence of carbohydrate fermentation results.
  • Blackening of the medium: Occurs in the presence of H2 
  • Gas production: Bubbles or cracks in the agar indicate the production of gas ( formation of CO2and H2)
Triple sugar iron agar. A, Acid slant/acid butt with gas, no H2S (A/A). B, Alkaline slant/acid butt, no gas, H2S-positive (K/A H2S+). C, Alkaline slant/alkaline butt, no gas, no H2S (K/K). D, Uninoculated tube.
Uses
  • The test is used primarily to differentiate members of the Enterobacteriaceae family from other gram-negative rods.
  • It is also used in the differentiation among Enterobacteriaceae on the basis of their sugar fermentation patterns.
  • It is use for compare with 

    Kligler's Iron Agar (KIA) of sucrose's microbial.



References
https://www.time2026end.com/
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