The action of selected monoterpenes as biofilm control agents and antibiotic resistance modifiers

Multidrug-resistant bacteria in biofilms are a growing public health threat, due to their resistance to conventional antibiotics. Phytochemicals are attractive candidates because of their structural diversity and ability to potentiate antimicrobial activity. This study investigated the antibiofilm and resistance-modifying effects of two monoterpenes, menthol and linalool, alone and in combination with ten antibiotics, against Escherichia coli and Staphylococcus epidermidis. Menthol exhibited MIC and MBC of 800 µg/mL against E. coli and the same MIC against S. epidermidis, while linalool showed MICs of 800 µg/mL and 400 µg/mL, respectively. Combination assays revealed enhanced activity of erythromycin with both monoterpenes against E. coli and of amoxicillin with menthol against S. epidermidis, although sessile cells were largely unaffected. When applied individually, both monoterpenes caused a 3-log reduction in culturable E. coli biofilm cells. The overall findings highlight the antibiofilm activity of linalool and, particularly, menthol, supporting their role as antibiotic adjuvants against biofilm-associated infections.