Effects of linalyl acetate, (-)-menthone, and myrtenol against bacterial strains carrying efflux pumps and toxicity against Drosophila melanogaster

This study aimed to evaluate the antibacterial activity of the monoterpenes linalyl acetate, (-)-menthone, and myrtenol against two bacterial strains with efflux pump mechanisms, namely Staphylococcus aureus 1199B and K2068, which carry the efflux pumps NorA and MepA, respectively. Additionally, the toxicity of these monoterpenes was assessed in the arthropod model Drosophila melanogaster. The broth microdilution method was used to perform Minimum Inhibitory Concentration (MIC) tests at subinhibitory concentrations (MIC/8) in combination with norfloxacin, ciprofloxacin, and ethidium bromide to evaluate their combined effects. Toxicity evaluation of linalyl acetate, (-)-menthone, and myrtenol on D. melanogaster was conducted using fumigation bioassays and negative geotaxis methods. The results were expressed as the mean of triplicate replicates. Two-Way ANOVA followed by Bonferroni's post hoc test was used for statistical analysis. The present study demonstrated that linalyl acetate, (-)-menthone, and myrtenol did not exhibit direct antibacterial activity against the SA-1199B and SA-K2068 strains, as their MIC was ≥ 1024 μg/mL. When combined with norfloxacin, none of the compounds reduced the antibiotic's MIC, and when combined with ethidium bromide, only myrtenol reduced the MIC. For the SA-K2068 strain carrying the MepA efflux pump, all compounds reduced the MIC when combined with ciprofloxacin, and only myrtenol reduced the MIC of ethidium bromide. These results suggest that myrtenol may act as an efflux pump inhibitor for both NorA and MepA. Regarding the toxicity test with D. melanogaster, (-)-menthone exhibited the highest mortality rate. Myrtenol and (-)-menthone showed a greater interference with the animals' flight performance.