Why isn't a pH indicator needed in lactose broth fermentation tubes for detecting coliform bacteria?

A pH indicator is not needed in lactose broth fermentation tubes for detecting coliform bacteria because the presence of coliforms is indicated by gas production from lactose fermentation rather than pH changes. Coliform bacteria, such as E. coli, ferment lactose, producing acid and gas; however, the gas bubbles trapped in the inverted Durham tube provide a clear, direct visual cue of fermentation. The focus is on the gas indicator rather than pH changes, simplifying the detection process.