Effect of fluoroquinolone concentration on selection of resistant mutants of Mycobacterium bovis BCG and Staphylococcus aureus

When Mycobacterium bovis BCG and Staphylococcus aureus were plated on agar containing increasing concentrations of fluoroquinolone, colony numbers exh ibited a sharp drop, followed by a plateau and a second sharp drop. The pla teau region correlated,vith the presence of first-step resistant mutants. M utants were not recovered at concentrations above those required for the se cond sharp drop, thereby defining a mutant prevention concentration (MPC). A C-8-methoxy group lowered the MPC for an N-1-cyclopropyl fluoroquinolone.