THE EFFECT OF DICLOXACILLIN AND FUSIDIC ACID ON THE EXTRACELLULAR ANDINTRACELLULAR KILLING OF STAPHYLOCOCCUS-AUREUS

The effect of dicloxacillin and fusidic acid used alone and in combina tion on the extracellular and intracellular killing of four isolates o f Staphylococcus aureus in the presence of serum was studied. At the e xtracellular level, dicloxacillin (8 mg/L) had a bactericidal effect o n all four isolates, whereas fusidic acid (64 mg/L) had a bacteriostat ic effect on two isolates and no effect on the two other isolates. Fus idic acid significantly inhibited the extracellular bactericidal effec t of dicloxacillin on two isolates. Intracellular killing was measured in human neutrophil granulocytes, Dicloxacillin (8 mg/L) significantl y increased the intracellular killing of all four isolates, while fusi dic acid (64 mg/L) significantly increased the intracellular killing o f three isolates, but the killing was significantly lower than that of dicloxacillin, When the antibiotics were combined the intracellular k illing of three of the isolates was significantly lower than that of d icloxacillin alone. The viability of the granulocytes and their abilit y to produce superoxide anion were not affected by the antibiotics. In conclusion, we found that the increased intracellular killing of S. a ureus by dicloxacillin was inhibited by fusidic acid.