Levofloxacin: serum bactericidal activity against methicillin-resistant Staphylococcus aureus isolates

The aim of this study was to assess the serum bactericidal activity (SBA) of levofloxacin compared with that of ofloxacin against methicillin-resistant Staphylococcus aureus (MRSA) isolates. Serum from 10 healthy volunteers (seven females, three males) was collected after a single oral dose of either levofloxacin (500 mg) or ofloxacin (400 mg). Subjects were allocated randomly to treatment after at least a 1 week interval between antibiotic regimens. Three well-defined MRSA strains were tested, each susceptible to levofloxacin and ofloxacin, with different levels of resistance to methicillin (HBD 456, HBD 3 and HBD 2; class 1, 2 and 3 Tomasz heterogeneous resistance, respectively) together with a methicillin-susceptible (MSSA) reference strain (S. aureus ATCC 25,923). SBA was tested in vitro by a microtitration method 15 min before dosing and at 1, 4, 8 and 12 h after drug absorption. Levofloxacin was significantly more bactericidal than ofloxacin against all strains of S. aureus tested (SBA > or = 1:2). An SBA was recorded for only a short period with ofloxacin, and thereafter only bacteriostatic activity remained. This study, therefore, confirms the superior activity of levofloxacin over that of ofloxacin against MSSA and MRSA.