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

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

Answer: (C)

After a tube shows presumptive positives in the lactose broth, the standard follow-up step is to confirm and differentiate the types of coliforms. A small amount, 0.1 mL, of that presumptive broth is inoculated into EC broth because this medium is specifically used to indicate fecal coliforms (such as E. coli). EC broth is selective for these organisms and, when incubated at warm temperature (about 44.5°C), fecal coliforms ferment lactose and produce gas. The appearance of growth with gas in EC broth confirms the presence of fecal coliforms, which is necessary for the MPN calculation of fecal contamination.

The other media mentioned play different roles in the testing workflow: they may be involved in other steps of presumptive or pointer confirmations, but the described 0.1 mL transfer into EC broth is the confirmatory step for fecal coliforms.