In vitro and in vivo activity of d-serine in combination with β-lactam antibiotics against methicillin-resistant Staphylococcus aureus

Abstract

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.

Keywords: Combination; MRSA; Synergistic effect; d-Serine; β-Lactams.

Graphical abstract

Figure 1

Time–kill curves against MRSA strains ATCC43300, N315, 0603 and 0850 for combination of d-Ser (20 mmol/L, equivalent to 1/25 MIC) with OXA (Panel A) and MEM (Panel B). For Panel A, the OXA doses were: 1/32 MIC for MRSA ATCC 43300, 1/4 MIC for MRSA N315, 1/32 MIC for MRSA 0603, 1/8 MIC for MRSA 0850. For Panel B, the MEM doses were: 1/4 MIC for MRSA ATCC43300, 1/2 MIC for MRSA N315, 1/2 MIC for MRSA 0603, 1/2 MIC for MRSA 0850.

Figure 2

Animal survival rates of OXA, MEM alone and in combination with d-Ser in murine systemic infection model. Panel A: OXA±d-Ser against MRSA N315; Panel B: OXA±d-Ser against MRSA ATCC 43300; Panel C: MEM±d-Ser against MRSA N315; Panel D: MEM±d-Ser MRSA 0850. The infection doses were 7.91×10³ CFU per mouse for panel A, 6.5×10⁵ CFU per mouse for panel B, 1.87×10⁴ CFU per mouse for panel C and 1.2×10⁵ CFU per mouse for panel D.

Figure 3

Efficacy of OXA, MEM alone and in combination with d-Ser in murine neutropenic thigh infection model caused by MRSA N315. Infection doses: 4.0 × 10⁵ CFU per thigh for panel A, 7.0 × 10⁵ CFU per thigh for panel B and 6.3 × 10⁵ CFU per thigh for panel C. Oneway-ANOVA test was used for statistical analysis. ^*P < 0.05, ^***P < 0.001.