g you are given three bacterial cultures: Pseudomonas aeruginosa, Shigella sonnei & Escherichia coli and you carry out a Triple Sugar Iron (TSI) assay for each 1. The tube for P. aeruginosa shows a pink butt, pink slant with no gaps or breaks in the medium and no black precipitate 2. The tube for S. sonnei shows a yellow butt, pink slant with no gaps or breaks in the medium and no black precipitate 3. The tube for E. coli shows a yellow butt, yellow slant with no gaps or breaks in the medium and no black precipitate Which of these organism can reduce glucose but not lactose and/or sucrose?

Test Tube 2 (S. sonnei)

Explanation:

Triple sugar iron agar test is the microbiological assay used to test the fermentation reaction performed by the bacteria. The fermentation process differs based on substrate and the acids formed as a byproduct.

The test is based on the byproduct formed as a result of fermentation. The test tube contains 1% lactose, 1% sucrose and 0.1 % glucose, ferrous sulphate, phenol red pH indicator.

When the bacteria ferment any of the three sugar, the medium turns yellow.

In test tube 1 - The tube shows pink slant, pink butt and no black precipitate which indicates that no fermentation took.

In test tube 2 - the tube shows pink slant, yellow butt and no black precipitate due to fermentation of Glucose only due to alkali acid reaction.

In test tube 3 - the tube shows yellow slant and yellow butt which shows that glucose and either lactose or sucrose has been fermented.

Thus, test tube 2 (S. sonnei) is the correct answer.