Quinupristin/dalfopristin (Q/D) and β-lactams interact positively against MRSA. Protein inhibitors produce a muropeptide signature in Staphylococcus aureus that correlates with their synergism with β-lactams. Antibacterial agents with other mechanisms of action do not produce these anomalies and do not synergise with β-lactams. A decrease in PBP2 expression by protein synthesis inhibitors would provide a rationale for this synergism against MRSA.