Aims The objective of the study was to evaluate the antimicrobial interactions between two volatile agents, Cinnamomum cassia essential oil (CCEO) and 8‐hydroxyquinoline (8‐HQ) against Staphylococcus aureus strains in liquid and vapour phases. Methods and Results In vitro antimicrobial effect of CCEO in combination with 8‐HQ was evaluated against 12 strains of S. aureus by broth volatilization chequerboard method. Results show additive effects against all S. aureus strains for both phases. In several cases, sums of fractional inhibitory concentration values of our test combinations were lower than 0·6, which can be considered as a strong additive interaction. Moreover, composition of CCEO was analysed by gas chromatography–mass spectrometry analysis. In the CCEO, 26 compounds in total were identified, where (E)‐cinnamaldehyde was the predominant compound, followed by cinnamyl acetate, α‐copaene, bornyl acetate and caryophyllene. Conclusions Results showed additive in vitro growth‐inhibitory effect of CCEO and 8‐HQ combination against various standard strains and clinical isolates of S. aureus. Significance and Impact of the Study This is the first report on antibacterial effect of 8‐HQ and CCEO combination in liquid and vapour phases. Results of the study suggest these agents as potential candidates for development of new anti‐staphylococcal applications that can be used in the inhalation therapy against respiratory infections.

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Combinatory action of antimicrobial agents such as essential oils (EOs) show to be an effective strategy to overcome the problem with increasing antibiotic resistance of microorganisms, including Staphylococcus aureus. The objective of this study was to evaluate in vitro antimicrobial interactions between Origanum vulgare and Thymus vulgaris EOs against various S.aureus strains in both liquid and vapor phases using the broth volatilization checkerboard method. Fractional inhibitory concentrations (FICs) were determined for both liquid and vapor phases, and the composition of EOs was analyzed by gas chromatography-mass spectrometry using dual-column/dual-detector gas chromatograph. Results of oregano and thyme EOs combination showed additive effects against all S. aureus strains in both phases. In several cases, sums of FICs were lower than 0.6, which can be considered a strong additive interaction. The lowest FICs obtained were 0.53 in the liquid phase and 0.59 in the gaseous phase. Chemical analysis showed that both EOs were composed of many compounds, including carvacrol, thymol, γ-terpinene, and p-cymene. This is the first report on oregano and thyme EOs interactions against S. aureus in the vapor phase. It also confirms the accuracy of the broth volatilization checkerboard method for the evaluation of combinatory antimicrobial effects of EOs in the vapor phase.

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