The medium is inhibitory to gram-positive bacteria and most non-enteric gram-negative bacteria, but is not toxic to the target species. Escherichia coli is significantly inhibited, and colonies that grow are blue.
Most other enteric species growing on the medium will also form blue colonies. Strains of all four Shigella species (sonnei, flexneri, boydii and dysenteriae) as well as some Aeromonas species (hydrophila, jandaei and tructi) grow as large or medium-sized orange-red colonies. Shigella can be preliminarily differentiated from Aeromonas with an oxidase test (Shigella is oxidase-negative, Aeromonas is oxidase-positive)