Which of the following tests best differentiates Shigella species from E. coli?

• A. Hydrogen sulfide, VP, citrate, and urease
• B. Lactose, indole, ONPG, and motility
• C. Hydrogen sulfide, MR, citrate, and urease
• D. Gas, citrate, and VP

Answer: (B)

The most effective way to differentiate Shigella species from E. coli is through a set of biochemical tests that highlight key metabolic differences, particularly focusing on lactose fermentation, indole production, ONPG activity, and motility.

Shigella is a non-lactose fermenter, meaning it cannot ferment lactose within the typical incubation time used in many tests. This characteristic distinguishes it from the majority of E. coli strains, which are lactose fermenters. Additionally, certain E. coli species exhibit indole production, which is a byproduct of tryptophan metabolism, while Shigella typically does not. The ONPG test checks for the ability to produce β-galactosidase, and while some Shigella can test positive, the overall lac-negative profile strengthens the differentiation from most E. coli.

Motility is another factor; E. coli is motile due to the presence of flagella, whereas Shigella is non-motile. This combination of tests—lactose fermentation, indole production, ONPG activity, and motility—makes the choice particularly effective in distinguishing between these two genera, providing a clear and reliable means of differentiation in a laboratory setting.