In vitro activity of fosfomycin alone and in combination against Staphylococcus aureus with reduced susceptibility or resistance to methicillin, vancomycin, daptomycin or linezolid.

OBJECTIVES

To evaluate the activity of fosfomycin against a group of MRSA strains, including isolates with reduced susceptibility or resistance to vancomycin, daptomycin, linezolid and ceftaroline and to determine the effect of combining various combinations of antimicrobial agents used in the therapy of serious Gram-positive infections.

METHODS

Broth microdilution testing was used to determine the MICs of fosfomycin, vancomycin, daptomycin, linezolid, ceftaroline and cefazolin. Isolates were selected for further evaluations to determine in vitro synergy between fosfomycin and select antimicrobial agents using chequerboard broth microdilution testing. Fosfomycin was tested in combination with vancomycin, linezolid, daptomycin, ceftaroline and cefazolin.

RESULTS

Fosfomycin maintained activity against 100% of strains of vancomycin-resistant Staphylococcus aureus (VRSA) and linezolid-resistant S. aureus (LRSA), 86% of VISA and 95% of daptomycin-resistant S. aureus (DRSA) strains. The combination of fosfomycin with ceftaroline consistently demonstrated synergy among all 18 isolates against the strains tested. The next most potent combination regimen was linezolid with fosfomycin, which demonstrated synergy in 16 of the 18 strains. Daptomycin demonstrated synergy in only 7 of the 18 strains tested when combined with fosfomycin. Cefazolin demonstrated synergy in 6 of 6 strains and vancomycin demonstrated no interaction in 6 of 6 strains tested.

CONCLUSIONS

Fosfomycin demonstrated excellent activity against MRSA as well as isolates with resistance or reduced activity to other anti-MRSA drugs including vancomycin, daptomycin and linezolid. When combined with linezolid or daptomycin, fosfomycin demonstrated synergy for all or most strains tested. Thus, these combinations may have potential clinical utility when treating patients with serious infections caused by MRSA.