Question

after 48hours, a student observes a tube that has both yellow spant and yellow butt. if a yellow color in the butt indicates glucose fermentation, why doesn't the yellow slant ALSO indicate glucose fermentation?

Answer 1

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