In the b-galactosidase assay, it is important to measure the density of e. coli cells because select one:
a. the lac operon is induced only in a small percentage of cells in a culture.
b. the transformation efficiency of e. coli cells is low.
c. e. coli densities vary among cultures and with time, which will affect estimates of b-galactosidase production rates.
d. onpg production by e. coli is a function of cell density.
e. we are measuring per cell cleavage of x-gal in relation to culture conditions and time.

Option C, E. coli densities vary among cultures and with time, which will affect estimates of b-galactosidase production rates.

Explanation:

The activity of β-galactosidase  depends on the optical density or the intensity of the color.

In regards to color, yellow color intensity signifies the qualitative parameter of the β-galactosidase activity while the readings obtained through colorimeter is quantitative parameter of the β-galactosidase activity. In the presence of lactose , the gene responsible for producing β-galactosidase in E. coli is either induced or switched off.

Hence , option D  is correct