Assessing the Emergence of Resistance in vitro and Invivo: Linezolid Combined with Fosfomycin Against Fosfomycin-Sensitive and Resistant .

Purpose: We aimed to evaluate the synergistic effect of linezolid and fosfomycin on fosfomycin-sensitive and -resistant clinical isolates in vitro and in vivo and whether the emergence of fosfomycin resistance in is associated with changes in strain virulence, from the perspective of fitness cost.

Methods: The synergistic effect of linezolid and fosfomycin was studied via in vitro checkerboard and static time-kill assays, as well as based on the in vivo survival rate and hemolymph load of a infection model. Fosfomycin resistance was induced via a stepwise increase in concentration. Changes in the virulence of the strains after drug resistance were investigated using the infection model and reverse transcription quantitative polymerase chain reaction (RT-qPCR). In vitro and in vivo growth curves and competitive experiments were used to study the fitness cost of the strain. Finally, a static time-kill assay was performed to explore the effect of the combined medication.

Results: In vitro and in vivo data showed that linezolid combined with fosfomycin had a good synergistic effect on treatment. The infection model and RT-qPCR data showed that the virulence of the resistant strains was weakened to varying degrees. A survival curve and competition experimental data showed that this was related to the fitness cost of strains while acquiring resistance and negatively impacted linezolid treatment; however, the combination still showed a good synergistic effect in drug-resistant strains.

Conclusion: Linezolid combined with fosfomycin had a synergistic effect on both fosfomycin-sensitive and -resistant strains. Strains incur fitness costs as they develop drug resistance, which leads to a decrease in virulence. There is an interaction between fitness cost, virulence, and drug resistance, which indirectly affects drug treatment.