Next we can ask whether levofloxacin is the best fluoroquinolone for treatment of S. pneumoniae with respect of resistance. Since fluoroquinolones with a methoxy moiety at the C-8 position show enhanced activity against resistant mutants with other bacteria., we carried out a comparison of moxifloxacin and levofloxacin for the ability to select resistant mutants by placing a large number of cells on agar plates containing a variety of fluoroquinolone concentrations. We found that the MPC for moxifloxacin was about 1/5 that of levofloxacin. In this case we estimated MPC as the concentration that allows no colony formation when at least 10¹⁰ cells are applied to an agar plate.