Carvacrol thymol formulation: bactericidal activity without detectable early resistance induction in Gram-positive and Gram-negative bacteria

AIMS: The emergence of antimicrobial resistance requires alternative antimicrobial strategies capable of controlling bacterial growth while minimizing the risk of resistance selection. This study aimed to evaluate whether combinations of natural bioactive compounds, particularly carvacrol and thymol, could provide bactericidal activity without detectable early resistance induction under short-term experimental conditions, while maintaining fibroblast biocompatibility. METHODS AND RESULTS: Phenolic monoterpenes, carvacrol and thymol, the aromatic aldehyde trans-cinnamaldehyde, t-CNM, and the phenylpropanoid eugenol were evaluated individually and in binary combinations against Staphylococcus aureus ATCC 25923 and Escherichia coli ATCC 25922 reference strains. Checkerboard assays identified carvacrol-thymol as the most consistent synergistic combination, reducing the concentrations required for bacterial inhibition. Based on minimum inhibitory concentration (MIC) and minimum bactericidal concentration determinations, this combination showed bactericidal activity at reduced concentrations compared with the individual compounds. Under 96 h subinhibitory exposure, the carvacrol thymol combination did not produce a detectable progressive increase in MIC values in the tested strains. In sequential exposure assays using E. coli, susceptibility to carvacrol and thymol was maintained after subsequent ciprofloxacin exposure, as indicated by stable or reduced MIC values. Cytotoxicity assays using human dermal fibroblasts indicated that the effective carvacrol-thymol concentrations preserved cell viability under the tested conditions. CONCLUSIONS: These findings indicate that carvacrol-thymol synergy provides bactericidal activity without detectable early resistance induction in the short-term exposure model used here. Although further studies using clinical and multidrug-resistant isolates, longer serial passaging, and mechanistic assays are required, the results support the potential of natural bioactive combinations as candidates for alternative antimicrobial strategies.