 A bacterium that is a non-lactose fermenter and ferments glucose, initially causes a yellow slant/yellow bottom (acid/acid reaction) after 8 hours but then converts to a red slant/yellow bottom after 24 hours (alkali/acid reaction).Whereas if it ferments both lactose and glucose, it results in a yellow/yellow tube and remains that way due to the large amount of acid produced in the reaction.Blackening of the bottom due to H2S production may mask the acid reaction (yellow) in the bottom of the tube.