TSI Agar

Technique

TSI
,(/,)/,(

Principle

Triple sugar iron (TSI) agar contains the three sugars, glucose, lactose and sucrose as well as phenol red to indicate fermentation and ferrous sulphate to demonstrate hydrogen sulfide production (H2S combines with ferrous sulphate to yield the black precipitate ferrous sulfide). The glucose concentration is one tenth of the concentration of lactose and sucrose in order that the fermentation of this carbohydrate alone may be detected. The small amount of acid produced by fermentation of glucose is oxidized rapidly, in the slant which will revert to alkaline; in contrast, under the lower oxygen tension of the butt, the acid reaction is maintained. . .

Cautions

To promote the alkaline condition in the slant FREE EXCHANGE OF AIR must be permitted through a loose closure. If the tube is tightly closed with a stopper or a screw cap an acid reaction (caused solely by glucose fermentation) will also involve the slant

Read the reaction after l8-24 hours since the results cannot be interpreted properly if the slants are incubated for more than 48 hours.

Method

Results

Reaction	Explanation
Acid butt (yellow), alkaline slant (red)	Glucose fermented Acid throughout medium, butt and slant
Lactose or sucrose or yellow	both fermented Gas bubbles in butt, medium sometimes split Aerogenic culture
Blackening in the butt	Hydrogen sulfide produced lkaline slant and butt
(medium entirely red	None of the three sugars fermented

Positive control

Negative control

Reagents

Reference