4-Hydroxycinnamic acid and Itaconic acid exhibit broad spectrum synergy with conventional antibiotics in inhibiting multidrug-resistant Salmonella Typhimurium

One approach to combating the surge in bacterial antimicrobial resistance is to use adjuvants that sensitize drug-resistant strains to conventional antibiotics. Here, we evaluated 24 pure bioactive compounds for their antibiotic adjuvant activity against a multidrug-resistant strain of Salmonella Typhimurium (DT104). All 24 bioactive compounds exhibited measurable antibacterial activity against DT104, with MIC values ranging from 0.036 to 4 mg/mL. Moreover, four bioactive compounds, namely p-coumaric acid, 4-hydroxycinnamic acid, itaconic acid, and cinnamic acid, sensitized DT104 to five antibiotics: ampicillin, chloramphenicol, streptomycin, sulfamethoxazole, and tetracycline. Bioactive compounds, specifically 4-hydroxycinnamic acid and itaconic acid, induced the largest reductions in antibiotic MICs (1024- and 256-fold, respectively). Besides, 4-hydroxycinnamic acid was also synergistic with ciprofloxacin and inhibited the growth of ciprofloxacin-resistant strains of foodborne Salmonella (16-fold). The broad antibiotic adjuvant efficiency of 4-hydroxycinnamic acid was also demonstrated against multidrug-resistant strains of Escherichia coli and Klebsiella pneumoniae. Subsequent checkerboard analysis revealed that bioactive compounds maintained their antibiotic adjuvant activity even at their 0.5 × MIC levels. Subsequent time-kill kinetic studies confirmed the results of the checkerboard analysis. Overall, we demonstrate that 4-hydroxycinnamic and itaconic acid can sensitize drug-resistant bacteria to multiple antibiotics and can function as highly effective antibiotic adjuvants.