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

The best test for differentiating Shigella species from E. coli involves examining lactose fermentation, indole production, ONPG (ortho-Nitrophenyl-β-galactoside) hydrolysis, and motility.

Shigella species are typically non-motile and do not ferment lactose, which distinguishes them from many strains of E. coli that are motile and can ferment lactose. Specifically, while certain E. coli strains can rapidly utilize lactose, resulting in gas production and acid, Shigella does not produce gas from lactose fermentation, nor can it utilize lactose. The indole test helps in identifying specific strains of E. coli—some of which are indole-positive—while Shigella is generally indole-negative. ONPG tests for the ability to utilize lactose through a non-standard fermentation pathway, helping to confirm Shigella's lactose-negative status.

Option A includes hydrogen sulfide, VP (Voges-Proskauer), citrate, and urease tests, which are less direct in differentiating these organisms regarding lactose utilization, an essential factor in this context. Option C also includes hydrogen sulfide and citrate, which again are less specific for lactose fermentation. Option D features gas production, which E