Shigella sonnei is differentiated from other Shigella species by which test?

• A. A positive ONPG
• B. A positive phenylalanine deaminase reaction
• C. Its negative oxidase reaction
• D. Its ability to demonstrate motility at 22°C

Answer: (A)

The test focuses on differences in lactose utilization within the Shigella genus, detected through the presence of β-galactosidase. Shigella species are generally non-lactose fermenters, but Shigella sonnei uniquely has functional β-galactosidase activity that can enable lactose metabolism. The ONPG test uses o-nitrophenyl-β-D-galactopyranoside, which is cleaved by β-galactosidase to produce a yellow color. A positive ONPG result means the organism has β-galactosidase, indicating lactose metabolism capability; this helps distinguish Shigella sonnei from other Shigella species that lack this enzyme and remain ONPG negative. So, a positive ONPG result identifies Shigella sonnei in this context, whereas the others do not show this enzyme activity.