Promising Phytochemicals that Show Antibiofilm Activity at Sub-Minimum Inhibitory Concentrations: Trans-Cinnamaldehyde, Limonene, Eugenol, and Curcumin

As the challenges in the treatment of infections caused by multi-drug resistant microorganisms with well-known antimicrobial agents become a serious treat for the human health in worldwide, development of novel antimicrobials with potent antimicrobial activity has garnered significant attention. Therefore, this study aimed to investigate the antimicrobial and antibiofilm effects of four phytochemicals (trans-cinnamaldehyde, limonene, eugenol, and curcumin) against Gram-positive and Gram-negative bacteria and a yeast. Prior to antibiofilm assays, minimum inhibitory concentrations (MIC), minimum bactericidal concentrations, and minimum fungicidal concentrations were determined, with significant bactericidal and fungicidal effects being observed at low phytochemical concentrations. Also, biofilm inhibition efficiency of these phytochemicals was assessed at sub-MIC values (0.5x, 0.25x, and 0.125x MIC). At least 60% biofilm inhibition was observed for most of the microorganisms at the lowest tested concentrations (0.125x MIC) of the phytochemicals. Their biofilm inhibition capacity generally increased up to 80-90% depending on the concentration. Six data-driven models and their joint optimization adopted in this study yielded validation-based high predictive accuracy and identified optimal conditions.