Final answer:
Organisms that cannot use lactose as a carbon source would not show a yellow color change in the butt of the KIA slant, indicating a negative result for lactose fermentation.
Step-by-step explanation:
If an organism cannot use lactose as a carbon source, one would not expect to see acid production from lactose fermentation in the KIA (Kligler's Iron Agar) results. Specifically, in the KIA test, a color change to yellow indicates acid production from fermentation of the carbohydrate present in the medium. However, an organism that cannot utilize lactose will not produce acid from lactose; therefore, the butt of the slant will remain red, indicating no fermentation of lactose. This is in contrast to a lactose-fermenting organism, which would turn the butt of the slant yellow due to acid production.
More context to the question can be found by looking at the work of scientists like Jacob and Monod, who isolated different E. coli mutants unable to grow on lactose. Mutants that could not produce enzymes such as B-galactosidase, permease, or transacetylase lacked the capacity to metabolize lactose. In the KIA test, these mutants would not produce a yellow color change, indicating a negative result for lactose fermentation.