Which biochemical tests would differentiate between Shigella spp. and E. coli given their similarities?

To effectively differentiate between Shigella spp. and Escherichia coli, the selected biochemical tests should provide clear distinguishing characteristics of both bacteria. The combination of TSI (Triple Sugar Iron agar), motility, indole, and urease tests offers a strategic approach for identification.

The TSI test helps assess carbohydrate fermentation ability, as Shigella typically ferments glucose but does not produce gas or hydrogen sulfide (H2S), while certain strains of E. coli can produce gas from glucose and may show H2S production in TSI. The motility test is critical because Shigella is non-motile, while many strains of E. coli are motile, providing an immediate differentiation criteria. The indole test, which determines the ability to convert tryptophan into indole, can be used to identify E. coli, with some strains testing positive. Conversely, many Shigella species are indole negative. The urease test is valuable as Shigella does not produce urease, which allows the identification of these organisms based on their urease-negative reaction.

By combining these tests, the biochemical profile obtained will effectively establish the differentiation between Shigella and E. coli, leveraging their specific metabolic capabilities