Evaluation of the Quantitative and Structural Antimicrobial Activity of Thymol, Terpinen-4-ol, Citral, and E-2-Dodecenal, Antibiotic Molecules Derived from Essential Oils

Background/Objectives: This research was conducted to evaluate the antimicrobial activity of four molecules present in essential oils (thymol, terpinen-4-ol, citral, and E-2-dodecenal), complementing the study with the observation of structural damage caused by the contact of these compounds with microorganisms. Methods: The micro dilution in plates method was used to determine the minimum inhibitory concentration, using different concentrations of metabolites in contact with the microorganisms. Optical Microscopy was used to observe structural damage in yeasts, while Scanning Electron Microscopy (SEM) was used for bacteria. Results: In determining the minimum inhibitory concentration, very good activity was observed for all microorganisms at concentrations below 500 µg/mL or 0.05% w/w. In microscopic tests, we can observe three consequences of contact with the molecule to a greater or lesser extent. First, there is a clear decrease in the concentration of microorganisms. Second, we observe damage to the cell membrane. Finally, there are structural changes within the cytoplasm. Conclusions: This study demonstrated that the four metabolites possess good antimicrobial activity, in some of the tests they were even very close to the control antibiotics’ activity. Structural observations show that the activity can be explained by several factors. Many essential oils contain some of the molecules used, so their presence in nature could be a marker of antimicrobial activity.