What is the most distinguishing feature between E. coli and Shigella based on their biochemical reactions?

The most distinguishing feature between E. coli and Shigella based on their biochemical reactions is the presence of gas in Triple Sugar Iron (TSI) agar. E. coli typically ferments glucose, lactose, and/or sucrose, producing gas as a byproduct. This gas production is evident in TSI agar, where it can be observed as gas bubbles or cracks in the agar.

In contrast, Shigella does not produce gas during fermentation, which is a key characteristic that aids in differentiating it from E. coli. Despite both being Gram-negative, non-lactose fermenting, and belonging to the Enterobacteriaceae family, this distinction is particularly useful in laboratory diagnostics. The ability to produce gas is a result of the fermentation of sugars in the TSI medium, and this metabolic difference is fundamental in microbiological identification and classification of these organisms.