Antimicrobial synergism and antibiofilm activities of Mentha piperita L. and Eucalyptus globulus essential oils against Gram-positive cocci

Abstract

The development of new compounds with antimicrobial and antibiofilm activity is an urgent priority in One Health. Gram-positive cocci (GPC) infections deserve attention due to their capacity to cause severe and potentially fatal infections. The aim of this study was to evaluate the antimicrobial and antibiofilm activity of essential oils (EO) from Mentha piperita L. and Eucalyptus globulus and their combinations against Staphylococcus aureus and Enterococcus faecalis. The antimicrobial potential was evaluated by the disk diffusion method, and the Minimum Inhibitory Concentration (MIC) was determined by broth microdilution. EO combination assays were performed by the checkerboard method, which determines the Fractional Inhibitory Concentration (FIC) and classifies the association as synergistic, additive, or antagonistic. The antibiofilm activity of isolated EO and in combination was evaluated by crystal violet. The disk diffusion test confirmed the ability of EO to inhibit bacteria in their planktonic form. The MICs of E. globulus against S. aureus and E. faecalis were 1.25% and 0.15%, respectively, and for M. piperita L. it was 0.15% for both GPC, with bactericidal effect. The combination of EO showed synergistic activity for S. aureus (FIC = 0.375) and additive for E. faecalis (FIC = 0.750). Both GPC are moderately adherent biofilm formers. After addition of EO and their combination, the biofilm was reclassified as weakly adherent. These results indicate that both isolated and combined EO from E. globulus and M. piperita L. are capable of inhibiting the initial cellular adhesion of GPC.