Synergistic antibacterial activity and prevention of drug resistance of daptomycin combined with fosfomycin against methicillin-resistant Staphylococcus aureus

Abstract

The combination of daptomycin and fosfomycin is attracting attention due to the rising resistance observed in methicillin-resistant Staphylococcus aureus (MRSA). Most studies indicate that this combination exhibits synergistic or additive antimicrobial activity against MRSA. However, its capacity to prevent resistance remains uncertain. In this study, we first investigated the antibacterial effect of daptomycin and fosfomycin on MRSA, followed by predicting MRSA resistance under the influence of different drugs using a resistance mutation selection window. Subsequently, we conducted preliminary analyses of drug resistance gene mutations in resistance mutants through gene sequencing technology. Additionally, we established an in vitro biofilm infection model to explore the internal tolerance associated with phenotypic alterations in biofilms and assess the combination's ability to prevent drug resistance. Combination drug sensitivity experiments demonstrated that daptomycin and fosfomycin synergistically enhanced antimicrobial effects against the tested strains. This combination reduced the mutant prevention concentration of each drug and narrowed the selection window for drug-resistant mutations. Sequencing results indicated that specific resistance genes were mutated in the single-drug mutants, while no mutations were detected in the combination. Furthermore, the combination exhibited stronger inhibition and removal of biofilm compared to single agents. In conclusion, the daptomycin-fosfomycin combination displays a synergistic antibacterial effect against MRSA and shows enhanced capacity to prevent bacterial resistance, likely attributed to its ability to inhibit resistance gene mutations and exhibit superior anti-biofilm activity.

Keywords: antibiotics resistance; daptomycin; fosfomycin; methicillin-resistant Staphylococcus aureus.

Fig 1

Static-concentration time-kill studies displaying the activity of daptomycin, fosfomycin, and their combination against MRSA. ATCC 43300 (A), NO.5 (B), NO.6 (C), and NO.7 (D). CFU, colony-forming units; DAP, daptomycin; FOF, fosfomycin; Control: no drug; 1/2, 1/2 × MIC; LOQ: quantification limit.

Fig 2

Growth curves of MRSA biofilm substrates. Abbreviations: OD, optical density.

Fig 3

Inhibitory effect of daptomycin and fosfomycin on biofilm (A) and clearance of mature biofilm (B). Data represent the average of three independent experiments (mean ± SD). *** , P-value ≤0.001. DAP, daptomycin; FOF, fosfomycin; Control: no drug; 1/2, 1/2 × MIC.

Fig 4

Scanning electron microscopy of biofilm morphology in each group. Control (A), Daptomycin (B), Fosfomycin (C), Daptomycin-fosfomycin combination (D). DAP, daptomycin; FOF, fosfomycin; Control, no drug.

Fig 5

Survival of mice at 72 hours of different treatment regimens in a mouse model of bacteremia infection. DAP, daptomycin; FOF, fosfomycin; Control, no drug.

Fig 6

Effect of antimicrobial therapy on the clearance of MRSA strains in blood and spleen. Changes (mean ± s.d.) in blood (A) log₁₀ CFU/mL and kidney (B) log₁₀ CFU/g relative to baseline in mice infected with MRSA strains (untreated mice, n = 6); control (n = 6) and daptomycin, fosfomycin, and combination-treatment groups (n = 6 in each group). ***, P-value ≤ 0.001. CFU, colony-forming units; DAP, daptomycin; FOF, fosfomycin; Control, no drug.