The antibacterial activity and efflux pump reversal of thymol and carvacrol were investigated against the Staphylococcus aureus IS-58 strain in this study, as well as their toxicity against Drosophila melanogaster. The minimum inhibitory concentration (MIC) was determined using the broth microdilution method, while efflux pump inhibition was assessed by reduction of the antibiotic and ethidium bromide (EtBr) MICs. D. melanogaster toxicity was tested using the fumigation method. Both thymol and carvacrol presented antibacterial activities with MICs of 72 and 256 µg/mL, respectively. The association between thymol and tetracycline demonstrated synergism, while the association between carvacrol and tetracycline presented antagonism. The compound and EtBr combinations did not differ from controls. Thymol and carvacrol toxicity against D. melanogaster were evidenced with EC₅₀ values of 17.96 and 16.97 µg/mL, respectively, with 48 h of exposure. In conclusion, the compounds presented promising antibacterial activity against the tested strain, although no efficacy was observed in terms of efflux pump inhibition.

The minimum inhibitory concentration (MIC) is used to define the lowest concentration at which a substance can inhibit bacterial growth. This study aimed to evaluate the MIC of pyrogallol against Staphylococcus aureus and to propose a method for building growth inhibition curves of bacterial strains from MIC assays. S. aureus strains 1199B (NorA) and 1199 (wild type) were used for the assays. Pyrogallol MIC tests were performed by the broth microdilution method. The proposed method uses RGB images of the microdilution plate using the R (Red), G (Green), and B (Blue) channels to extract information for the construction of the bacterial growth inhibition curve (GIC). Pyrogallol demonstrated a MIC of 512 µg/mL against the two S. aureus strains tested. The GIC was calculated and the MIC point of pyrogallol was identified against the tested strains. The proposed method suggested the same MIC point for pyrogallol when using microplate images before and after the addition of resazurin. Through this methodology, the subjectivity of visual analysis in MIC tests can be eliminated.