Correct option is C
Explanation-
The LacZ gene encodes β-galactosidase, an enzyme that hydrolyzes lactose into glucose and galactose. It also acts on chromogenic substrates like X-gal and ONPG, used to visualize LacZ activity.
Correct options-
Option A - E. coli growth on LB agar with X-gal results in blue colored colonies because LacZ produced in the cell hydrolyzes X-gal present in the medium into a blue product.
X-gal is cleaved by β-galactosidase (LacZ) into 5-bromo-4-chloro-indoxyl, which spontaneously dimerizes to form a blue product. It is commonly used in blue-white screening.
Option B- When the membranes of cells harboring LacZ are permeabilized and incubated in a buffer with ONPG, the solution turns yellow because LacZ encoded protein hydrolyzes ONPG.
ONPG (ortho-nitrophenyl-β-galactoside) is hydrolyzed by LacZ to ortho-nitrophenol, which is yellow. It is commonly used in LacZ activity assays.
Option D - E. coli growth on MacConkey agar results in pink colonies due to shift in pH of the medium.
Acid production drops pH, causing neutral red to turn pink. This indicates lactose fermentation (via LacZ activity).
Incorrect options-
Option C - E. coli growth on MacConkey agar results in pink colonies because LacZ encoded protein hydrolyzes the neutral red dye into a pink colored product.
MacConkey agar contains lactose and neutral red (a pH indicator, not a substrate).
Mechanism:
If E. coli ferments lactose, it produces acid. The pH drops, turning neutral red dye pink due to acidic pH — it is not hydrolyzed by LacZ.
LacZ hydrolyzes lactose, not the dye.
Final Answer: Option C
Option C is incorrect, because it falsely claims that LacZ hydrolyzes the dye (neutral red), whereas the color change is due to pH shift, not dye hydrolysis.
