In the multiple-tube fermentation method, as presumptive positives are inoculated into BGB, 0.1 mL of presumptive broth is also transferred into which broth?

• A. Lauryl Tryptose Broth (LTB)
• B. Brilliant Green Broth (BGB)
• C. EC broth
• D. Lactose broth

Answer: (C)

The test uses a confirmatory step after the presumptive positives show lactose fermentation in the initial broth. A small aliquot from each presumptive-positive tube is placed into a selective medium that specifically checks for E. coli. EC broth is designed to favor growth of E. coli while inhibiting many other bacteria, and it detects lactose fermentation with gas production, which is characteristic of E. coli. Transferring a 0.1 mL portion into EC broth provides a controlled inoculum that allows this targeted confirmation to occur reliably. The Brilliant Green Broth is used as another confirmatory medium for coliforms in general, not specifically for identifying E. coli, and the other options are part of earlier steps or not the intended confirmatory medium here.