Synergistic activity of fosfomycin and flucloxacillin against methicillin-susceptible and methicillin-resistant Staphylococcus aureus: in vitro and in vivo assessment

Fosfomycin (FOF) exhibits broad-spectrum antimicrobial activity, and is mainly used in combination therapy. Previous in vitro studies have shown synergistic effects of FOF in combination with flucloxacillin (FLX) against Staphylococcus aureus isolates. This study aims to validate these findings in vitro and investigate the synergistic effect in an in vivo Galleria mellonella model. Five methicillin- and FOF-susceptible isolates (ATCC-29213 & 4 clinical isolates); one methicillin- and FOF-resistant strain (DSMZ-23622) and four methicillin-resistant and FOF-susceptible strains (ATCC-33592 & 3 clinical isolates) were tested with checkerboard assays to assess synergism. Time-kill curves were generated for two MSSA (ATCC 29213 and 231/20) and two MRSA strains (ATCC 33592 and DSMZ 23622). The in vivo efficacy of FOF and/or FLX was evaluated by a G. mellonella survival assay and by determining the total bacterial count (TBC) in hemolymph. Checkerboard assays revealed additive or indifferent effects, with some indicating synergism. Time-kill curves demonstrated higher reduction in TBC with combination therapy compared to monotherapy. In vivo, the combination therapy showed the greatest reduction of TBC in larval haemolymph compared to monotherapy, and the survival assay showed highly synergistic activity of FLX plus FOF against MRSA (ATCC-33592) and MSSA (ATCC 6538), resulting in an average reduction in mortality of 48 and 40%, respectively, compared to monotherapies. Therefore, FOF plus FLX could be an alternative for the calculated or definitive treatment of S. aureus infections without antimicrobial susceptibility results or even for salvage therapy of MRSA infections after treatment failure or necessary discontinuation of classical MRSA drugs.