How could the procedures used in this experiment be altered to measure bacteriostatic effects?

A bacteriostatic chemical agent means that the chemical blocks the growth of the bacteria. A bactericidal chemical means that the chemical agent kills the bacteria.
To test whether a chemical agent is bacteriostatic, the researcher should, first of all, make a culture with the bacteria of the experiment. Then, the diluted chemical tested for its bacteriostatic effect should be added to the bacterial culture and this mixture should be left to incubate overnight. The next day, the researcher should see if there is any growth of bacteria. If no bacterial growth is observed, then the next step should be a subculture. Meaning that the bacteria should be cultured again to a sterile medium that does not have the chemical agent tested. After incubating this subculture again, the researcher should see whether there is any bacterial growth. If there are no bacteria in this subculture, then the chemical killed all the bacteria and this means that it was bactericidal. If the incubation of the subculture has bacteria, then the chemical did not kill the bacteria in the first place (it only stopped their growth) and it means that it was bacteriostatic. 

They became more resistant.