In an experiment, a researcher wanted to investigate the effect of different concentrations of a drug on the growth of bacteria. The researcher grew bacterial cultures in a nutrient broth and added different concentrations of the drug. The cultures were incubated for 24 hours, and the optical density was measured at 600 nm. The results showed that the higher the concentration of the drug, the lower the optical density.

What conclusion can be drawn from the results of this experiment?