Which statement about Shigella sonnei is true?

• A. Large numbers of organisms must be ingested to produce disease.
• B. H2S positive
• C. Produces red colonies on Salmonella-Shigella agar after 18 hours of incubation
• D. The organism is a delayed lactose fermenter.

Answer: (D)

Shigella sonnei is notable for causing disease with a very small infectious dose, due to its ability to invade the intestinal mucosa. This means ingested numbers can be quite low and still cause illness, so the idea that large numbers are required is not correct. In the lab, Shigella does not behave as a rapid lactose fermenter. It generally shows little to no lactose fermentation, and when lactose metabolism is observed it tends to be weak or occur only after extended incubation—described in some texts as a delayed lactose fermenter. On Salmonella–Shigella agar, Shigella colonies are not red or pink (they don’t ferment lactose) and they do not produce hydrogen sulfide, which helps distinguish them from Salmonella. So the statement that Shigella sonnei is a delayed lactose fermenter best fits its typical, sluggish lactose fermentation profile, making it the true option among the choices.