Final answer:
Lactose nonfermenter bacteria form colorless or white colonies on both EMB and MacConkey agar plates due to the lack of lactose fermentation which is indicated by the absence of a pink or red pigment in colonies.
Step-by-step explanation:
Lactose nonfermenter bacteria appear as colorless or white colonies on both EMB (Eosin Methylene Blue) and MacConkey agar plates.
On a MacConkey agar plate, which is commonly used to differentiate bacteria based on their lactose fermentation capability, non-lactose fermenters like Serratia marcescens will not produce the pink or red pigment that lactose fermenters like Escherichia coli produce, resulting in colorless or white colonies against the contrasting agar color.
Similarly, the EMB agar, which is both selective and differential, contains indicator dyes like eosin and methylene blue that inhibit gram-positive growth and distinguish between lactose fermenting and non-lactose fermenting organisms; non-lactose fermenters will form colorless colonies.