Efflux pump inhibitors from plant compounds: A new strategy to modulate bacterial growth and morphology

Antidrug resistance remains a major therapeutic challenge, with combination therapies, often enhanced by plant-derived compounds, gaining popularity due to their increased efficacy in antibacterial treatments. This study focused on the effects of plant-derived efflux pump inhibitors (EPIs) on bacterial growth in liquid medium and the ability of cells incubated with selected inhibitors to form bacterial clusters on solid medium. Minimum inhibitory concentrations (MICs) of berberine, capsaicin, coumarin 6, curcumin, palmatine, and piperine were determined against P.mirabilis, E.coli, E.faecalis, and B.cereus using a modified resazurin assay. Antimicrobial activity was shown by palmatine, curcumin, and berberine. Literature studies confirmed that these compounds also belong to the Sortase A inhibitors. Next, how the addition of a specific concentration of EPI affects the change in optical density of the bacterial suspension over time was investigated in a bioreactor. The results confirm that the addition of EPI to the medium changes the characteristics of the growth curve, in particular the logarithmic phase. Digital holotomography was used to investigate EPI-induced changes in bacterial cluster growth dynamics. Median RI values of bacterial cells, volume of structures and dry mass were analyzed. Significant changes in the characteristics of clusters development on agar medium were observed between groups, including lack of cluster development, slowing of divisions, and elongation of cells. This study showed that palmatine, berberine, and curcumin could be promising substances for antimicrobial therapy, also as a component of combination therapy. In addition, they can find applications as preservatives used in the food, pharmaceutical and cosmetic industries.