Which broth is used in the MPn Confirmed Phase?

• A. Lauryl Tryptose Broth
• B. Brilliant Green Bile Broth
• C. MacConkey Broth
• D. Tryptic Soy Broth

Answer: (B)

In the MPn testing sequence, the Confirmed phase uses a selective, differential broth to verify lactose-fermenting coliforms identified in the presumptive phase. Brilliant Green Bile Broth contains bile salts and brilliant green dye to inhibit many non-coliform bacteria, while providing lactose as the fermentable substrate. If the organism is a coliform, lactose fermentation during incubation produces gas, which is observed in the Durham tube and confirms the presence of coliforms. That gas-positive reaction is what makes this broth the choice for confirmation.

The other broths serve different roles: Lauryl Tryptose Broth is used in the presumptive phase to screen for gas production from lactose fermentation; MacConkey Broth can differentiate lactose fermenters but isn’t the standard Confirmed phase medium in the MPn method; Tryptic Soy Broth is a non-selective enrichment medium and isn’t used for confirming lactose-fermenting coliforms.