[In vitro activity of linezolid, moxifloxacin, levofloxacin, clindamycin and rifampin, alone and in combination, against Staphylococcus aureus and Staphylococcus epidermidis].

Information about the in vitro effect of combinations of anti-staphylococcal agents on staphylococci is scarce. The aim of the study was to evaluate the in vitro activity of linezolid, moxifloxacin, levofloxacin, clindamycin and rifampin, alone or in combination, against Staphylococcus spp. Two Staphylococcus aureus and two Staphylococcus epidermidis strains isolated from blood cultures were studied using the killing curve method. The combinations analyzed were linezolid+moxifloxacin, linezolid+levofloxacin, linezolid+clindamycin, linezolid+rifampin, moxifloxacin+rifampin, moxifloxacin+clindamycin, levofloxacin+rifampin and levofloxacin+clindamycin. The following concentrations (mg/l) were used: 8 and 16 for linezolid, 2 for moxifloxacin, 3 for levofloxacin, 2 for clindamycin and 2 and 5 for rifampin. The activity was considered synergistic when a reduction in growth of at least 2 log(10) was produced with the combination in comparison to the most active antibiotic alone; antagonistic when a growth of at least 2 log(10) was produced with the combination in comparison to the most active antibiotic alone; and indifferent if the variation was less than 1 log(10). Linezolid and clindamycin were bacteriostatic, while moxifloxacin and levofloxacin were bactericidal. Rifampin was bacteriostatic against S. aureus and bactericidal against S. epidermidis. Linezolid and clindamycin reduced the bactericidal activity of levofloxacin and moxifloxacin, however an antagonistic effect was only observed against S. aureus. Other combinations of linezolid, rifampin, clindamycin, levofloxacin or moxifloxacin were indifferent. Linezolid and clindamycin antagonize the bactericidal activity of fluorquinolones against staphylococci. There was no difference between any other combinations against either S. aureus or S. epidermidis.