Objective: The purpose of this study is to assess the in vitro/vivo activities of linezolid plus fosfomycin against Staphylococcus aureus  (S. aureus ) isolates with varying susceptibility to the study drugs.
Methods: The increasing concentration stepwise method was used to induce S. aureus  resistant strains. The in vitro antibacterial activity of linezolid combined with fosfomycin against S. aureus  in vitro was studied by time-kill curve and PAE. The transmission electron microscopy (TEM) was employed to observe the cell morphology of bacteria treated with drug, and the changes of cell wall thickness were recorded. The Galleria mellonella  infection model was established to demonstrate the in vivo efficacy of linezolid and fosfomycin against S. aureus  with varying susceptibility.
Results: The antibiotic combination showed excellent synergistic or additive effects on the original and the linezolid-resistant strain, but showed indifferent effect for fosfomycin-resistant strain. TEM images showed that fosfomycin alone and in combined could reduce the cell wall thickness of the strains resistant to linezolid and cell lysis, while linezolid increases the cell wall thickness of the strains resistant to fosfomycin. In the Galleria mellonella  infection model, the survival rate of the antibiotic combined was improved compared with that of the single drug. There was a good correlation between in vivo efficacy and in vitro susceptibility.
Conclusion: The type of interaction expressed in the test combination was highly dependent on fosfomycin resistance.
Keywords: linezolid, fosfomycin, interaction, resistant, Staphylococcus aureus , Galleria mellonella