In the presence-absence test, the presence of total coliforms is indicated by the medium turning yellow and the formation of what?

• A. purple, pink, gas
• B. purple, yellow, gas
• C. brown, yellow, shiny yellow colonies
• D. brown, pink, shiny pink colonies

Answer: (A)

In the presence-absence test, total coliforms ferment lactose, which lowers the pH and turns the broth yellow. The key positive signal is gas production, detected as bubbles in the inverted Durham tube threaded into the medium. So when the medium is yellow and the Durham tube shows gas, that indicates a positive result for total coliforms. The other color patterns don’t align with the typical positive Fermentation signal in this test, whereas gas formation directly accompanies the yellow color as the fermentation byproduct.