Combined antibacterial activity of essential oils extracted from Lavandula maroccana (Murb.), Thymus pallidus Batt. and Rosmarinus officinalis L. against antibiotic-resistant Gram-negative bacteria

Abstract

IntroductionCombinations of plant essential oils (EOs) provide a novel approach to increasing effectiveness against resistant bacteria. In this context, the combined antibacterial effects of EOs obtained from Lavandula maroccana (Murb.), Thymus pallidus Batt., and Rosmarinus officinalis L. against three multi-drug resistant Gram-negative bacteria were evaluated. MethodsThe antibacterial effect of each EO alone was evaluated by agar diffusion and broth microdilution methods, while the effects of EO binary combinations were evaluated through the determination of the fractional inhibitory concentration index (FICI) using the checkerboard method. ResultsGC-MS analysis of EOs showed that T. pallidus was dominated by γ-terpinene, thymol and p-cymene, while the main constituents of R. officinalis were 1,8-cineole, camphor, and α-pinene. L. maroccana EO was characterized by a high percentage of carvacrol. EOs tested individually demonstrated moderate antibacterial activity with MIC and MBC values from 1.3 mg/mL to72 mg/mL, whereas the FICI of their binary combinations revealed interesting interactions. The combination of EOs obtained from T. pallidus and L. maroccanas showed the highest number of effective combinations, with synergistic effects against P. aeruginosa and K. pneumoniae, and an additive effect against E. coli. The mixture of L. maroccana and R. officinalis EOs showed synergistic effects against E. coli and P. aeruginosa with an additive effect against K. pneumoniae. The combination of R. officinalis and T. pallidus EOs showed a synergistic effect against K. pneumoniae only. ConclusionThese findings suggest that combining L. maroccana with T. pallidus and/or R. officinalis EOs may constitute a promising option for controlling the emergence of multidrug-resistant bacteria.