As D-amino acids play important roles in the physiological metabolism of bacteria,combination of D-amino acids with antibiotics may provide synergistic antibacterial activity.The aim of the study was to evaluate in vitro and in vivo activity of D-serine alone and in combination with β-lactams against methicillin-resistant Staphylococcus aureus (MRSA) strains,and to explore the possible sensitization mechanisms.The activity of D-serine,β-lactams alone and in combinations was evaluated both in vitro by standard MICs,time-kill curves and checkerboard assays,and in vivo by murine systemic infection model as well as neutropenic thigh infection model.An in vitro synergistic effect was demonstrated with the combination of D-serine and β-lactams against MRSA standard and clinical strains.Importantly,the combinations enhanced the therapeutic efficacy in the animal models as compared to β-lactam alone groups.Initial mechanism study suggested possible revision of D-alanine-D-alanine residue to D-alanine-D-serine in peptidoglycan by adding of D-alanine in the medium,which may cause decreased affinity to PBPs during transpeptidation.In conclusion,D-serine had synergistic activity in combination with β-lactams against MRSA strains both in vitro and in vivo.Considering the relatively good safety of D-serine alone or in combination with β-lactams,D-serine is worth following up as new anti-MRSA infection strategies.