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The Staphylococcus aureus bacteria is a pathogen considered opportunistic and that has been acquiring resistance to several classes of antibiotics, mainly due to the synthesis of efflux pumps, which are proteins that expel these drugs intracellularly, reducing their effectiveness. The objective of this study was to evaluate the ability of isoeugenol to inhibit S. aureus efflux pumps and to determine its toxicity against a eukaryotic model (Drosophila melanogaster). IS-58, K2068 and K4414 S. aureus strains were used in the study. Isoeugenol minimum inhibitory concentration (MIC) and antibiotic modulation were evaluated in efflux pump inhibitory tests as well as in ethidium bromide (EtBr) assays. Toxicity tests against D. melanogaster assessed mortality and negative geotaxis. Isoeugenol obtained a relevant MIC result and a synergism was observed when isoeugenol was associated with the antibiotics, mainly with ciprofloxacin. Isoeugenol was able to affect all three efflux pumps tested, especially in strain K4414. The mortality of D. melanogaster caused by isoeugenol administration started after 12 h of exposure, being volume dependent and having an LC50 of 81.69 μL/L. In the negative geotaxis test, a statistical difference was observed after 24h of exposure compared to the control, demonstrating that damage to the locomotor apparatus had occurred. Based on the results, isoeugenol is a putative efflux pump inhibitor, becoming an alternative in blocking these proteins, and demonstrated acute toxicity against D. melanogaster.

Among the bacterial resistance mechanisms, efflux pumps are responsible for expelling xenobiotics, including bacterial cell antibiotics. Given this problem, studies are investigating new alternatives for inhibiting bacterial growth or enhancing the antibiotic activity of drugs already on the market. With this in mind, this study aimed to evaluate the antibacterial activity of Estragole against the RN4220 Staphylococcus aureus strain, which carries the MsrA efflux pump, as well as Estragole's toxicity in the Drosophila melanogaster arthropod model. The broth microdilution method was used to perform the Minimum Inhibitory Concentration (MIC) tests. Estragole was used at a Sub-Inhibitory Concentration (MIC/8) in association with erythromycin and ethidium bromide to assess its combined effect. As for Estragole's toxicity evaluation over D. melanogaster, the fumigation bioassay and negative geotaxis methods were used. The results were expressed as an average of sextuplicate replicates. A Two-way ANOVA followed by Bonferroni's post hoc test was used. The present study demonstrated that Estragole did not show a direct antibacterial activity over the RN4220 S. aureus strain, since it obtained a MIC ≥1024 μg/mL. The association of estragole with erythromycin demonstrated a potentiation of the antibiotic effect, reducing the MIC from 512 to 256 μg/mL. On the other hand, when estragole was associated with ethidium bromide (EtBr), an antagonism was observed, increasing the MIC of EtBr from 32 to 50.7968 μg/mL, demonstrating that estragole did not inhibited directly the MsrA efflux pump mechanism. We conclude that estragole has no relevant direct effect over bacterial growth, however, when associated with erythromycin, this reduced its MIC, potentiating the effect of the antibiotic.