Carvacrol and Thymol, a Synergistic Antimicrobial Activity Against Bacterial and <i>Candida</i> Species

The success of antimicrobial compounds today is diminished by the ability of microbes to develop resistance. The use of natural compounds as alternatives to antibiotics and/or antifungals is increasing. The potential use of essential oils, particularly their phenolic compounds, thymol or carvacrol, is receiving great interest. The study aims to evaluate the antimicrobial activity of the two monoterpenes against Gram-positive and -negative bacteria and various Candida spp., in planktonic form, through MIC and MBC/MFC evaluation, and sessile growth form, through crystal violet and XTT assays. In addition, Checkerboard test evaluated synergistic effects between the phenolic compounds, and phenolic compounds and antibiotics/antimycotics substances, and cytotoxicity versus three different human cell lines was evaluated through MTT assay. Results obtained showed antimicrobial activity of the phenolic compounds against bacteria and Candida spp., in both planktonic and sessile cultures. The MIC/MBC range of carvacrol and thymol was 125-250 µg/mL against all strains tested, while for Candida spp., the MIC/MFCs ranged from 250 to 2000 µg/mL. Several combinations of monoterpenes/antimicrobial compounds showed synergistic or additive effects at concentrations lower than MIC values in both planktonic and sessile forms. Carvacrol was toxic to all the tested cell lines (concentrations > 31 µg/mL); whereas thymol was toxic to only two of the tested cell lines (human urinary cell line excluded). In conclusion, although a high level of cytotoxicity against different cell lines has been demonstrated, the additive and synergistic effects shown could allow their therapeutic use at nontoxic concentrations, encouraging further studies on these substances.