This is only to make sure that you are not missing out on any organisms. Usually this happens in case of fecal E.coli when some of them become some sort of auxotrophic mutants naturally. You must remember that the fecal coliform count should be nil or zero.

Lauryl Tryptose media contains sodium lauryl sulfate, which inhibiting gram-positive microorganisms, serves as a selective agent for coliforms. The addition of lactose to the medium allows for detection of rapid lactose fermentation by coliforms. Coliforms grown in Lauryl Tryptose Broth ferment lactose and produce gas. Other bacteria are either inhibited or grow without producing gas.

Brilliant Green Bile Lactose Broth is formulated for presumptive identification and confirmation of coliform bacteria. Ox-bile and Brilliant green inhibit the growth of gram-positive bacteria including lactose fermenting Clostridia (but not all gram positives). Brilliant Green Bile Lactose Broth is used for the confirmation of presumptive positive tubes showing gas production in Lactose Broth or Lauryl sulfate Broth because of some Gram-positive spore formers may also produce gas in the bile or Brilliant green media.

Thanks for the response. Just to follow up a bit more about the discussion, does it mean there are non-coliform microorganism that can grow and produce gas in the bile or Brilliant green media (BGB) but not able to grow in Lauryl Tryptose (LTB) media? Or say it in another way, is it possible that you can have a false positive or false negative result if you inoculate your sample into LTB and BGB simultaneously instead of sequentially (LTB positive then BGB)?

Hello. Yes, you are right. You will have mostly false positive in BGB if you go directly to BGB. Several positives can be negative in LTB. So better way first LTB than BGB.

Good luck!