Tunicamycin and β-Lactams with Five Membered Ring Structures Have Stronger Synergistic Effects Against Gram-Positive Bacteria

Objective: The prevalence of antimicrobial resistance is increasing among gram-positive bacteria, particularly the resistance of methicillin-resistant Staphylococcus aureus (MRSA) to β-lactam antibiotics. Methods: Checkerboard minimum inhibitory concentration (MIC) assays, time-killing assays, enzyme inhibition assays, hemolysis tests, and cytotoxicity analyses were performed to confirm the synergistic effects of tunicamycin (TUN) in combination with β-lactam antibiotics at levels without potential cytotoxicity. Results: TUN at ≤ 1 μg/mL decreased the MICs of cephalosporins or penicillins against MRSA strain USA300, which became sensitive. TUN showed synergistic effects with cefuroxime sodium (FIC index ≤ 0.19) against tested gram-positive bacterial strains of both human clinical and animal origin. Time-killing assays indicated that the combination treatment eliminated MRSA 3–9 h after inoculation. Furthermore, we confirmed the better synergy of specific β-lactam antibiotics at potentially cytotoxicity-free concentrations in combination with TUN against β-lactam-resistant gram-positive microorganisms. Conclusion: This study provides an experimental basis for combination treatment with TUN and β-lactam antibiotics with specific chemical structures for the clinical treatment of gram-positive bacterial infections.