Antibiotic adjuvants have no or little antibiotic activity themselves but can enhance the activity of co-administered antibiotics, or reverse antibiotic resistance mechanisms. Now, Song, Liu et al. report a broad-spectrum antibiotic adjuvant that reverses resistance in multi-drug resistant (MDR) Gram-negative pathogens. The short linear antibacterial peptide known as SLAP-S25 carries four non-natural amino acids of 2,4-diaminobutanoic acid and was found to increase the efficacy of antibiotics in all major classes. SLAP-S25 potentiated the efficacy of antibiotics against diverse Gram-negative bacteria, including carbapenem-resistant and colistin-resistant Escherichia coli, and reversed resistance to MDR E. coli in animal models. SLAP-S25 binds to lipopolysaccharide and phosphatidylglycerol to induce membrane damage, which is required for synergy with antibiotics.