Combination of medicinal plants with antibiotics against Klebsiella pneumoniae and Acinetobacter baumannii

Background and aims: One essential plant-based strategy to deal with infections is antimicrobial synergism, which can make antimicrobials more efficient. The aim of this study was to investigate the interaction between these extracts and two widely used antibiotics, meropenem and gentamicin, on two multidrug-resistant (MDR) bacteria, K. pneumoniae and A. baumannii, in vitro. Methods: Different concentrations of Rosa damascena Mill., Malva sylvestris L., and Zataria multiflora Boiss. hydroalcoholic extracts (۲-fold serial dilution from ۱۳۱۰۷۲ to ۲۵۶ μg/mL) were administered against two MDR bacteria, and their combination with gentamicin and meropenem (serial dilution from ۳۲ to ۰.۰۱۵ μg/mL) was investigated by the resazurin-based microdilution and the checkerboard method. The phytochemical properties of the extracts were also examined, and the total phenolic content (TPC), total flavonoid content (TFC), anthocyanin content, and antioxidant capacity of the extracts were determined. Results: Z. multiflora and R. damascena showed high antibacterial activity, and their minimal inhibitory concentrations on Acinetobacter baumannii were ۱۰۲۴ and ۲۰۴۸ μg/mL, respectively. Z. multiflora also had high TFC, TPC, and antioxidant activity and demonstrated additive interaction with meropenem and gentamicin with fractional inhibitory concentrations of ۱ and ۰.۷۵, respectively. Conclusion: We suggest the potency of Z. multiflora-antibiotic combinations in treating MDR A. baumannii after future clinical studies.