do all bacteria ferment lactose? No.

Which bacteria does not ferment lactose?

Organisms unable to ferment lactose will form normal-colored (i.e., un-dyed) colonies. The medium will remain yellow. Examples of non-lactose fermenting bacteria are Salmonella, Proteus species, Yersinia, Pseudomonas aeruginosa and Shigella.

Why do some bacteria not ferment lactose?

The glucose and galactose will then be metabolized by the bacteria. Some bacteria are considered late lactose fermenters because they have the ability to ferment lactose by producing the intracellular enzyme but they lack beta-galactosidase-permease enzyme.