Antibacterial Activity of Terpenes and Terpenoids Present in Essential Oils

Abstract

Background: The antimicrobial activity of essential oils has been reported in hundreds of studies, however, the great majority of these studies attribute the activity to the most prevalent compounds without analyzing them independently. Therefore, the aim was to investigate the antibacterial activity of 33 free terpenes commonly found in essential oils and evaluate the cellular ultrastructure to verify possible damage to the cellular membrane. Methods: Screening was performed to select substances with possible antimicrobial activity, then the minimal inhibitory concentrations, bactericidal activity and 24-h time-kill curve studies were evaluated by standard protocols. In addition, the ultrastructure of control and death bacteria were evaluated by scanning electron microscopy. Results: Only 16 of the 33 compounds had antimicrobial activity at the initial screening. Eugenol exhibited rapid bactericidal action against Salmonella enterica serovar Typhimurium (2 h). Terpineol showed excellent bactericidal activity against S. aureus strains. Carveol, citronellol and geraniol presented a rapid bactericidal effect against E. coli. Conclusions: The higher antimicrobial activity was related to the presence of hydroxyl groups (phenolic and alcohol compounds), whereas hydrocarbons resulted in less activity. The first group, such as carvacrol, l-carveol, eugenol, trans-geraniol, and thymol, showed higher activity when compared to sulfanilamide. Images obtained by scanning electron microscopy indicate that the mechanism causing the cell death of the evaluated bacteria is based on the loss of cellular membrane integrity of function. The present study brings detailed knowledge about the antimicrobial activity of the individual compounds present in essential oils, that can provide a greater understanding for the future researches.

Keywords: antimicrobial activity; bacteria; bactericidal; essential oil; terpenes; time kill kinetics.

Figure 1

Time-kill curves for the bacteria S. Typhimurium, S. aureus and E. coli. Control: bacteria untreated; MIC: minimum inhibitory concentration obtained by the MIC assay to each terpene.

Figure 2

Scanning Electron Microscopy of untreated E. coli (A) and E. coli treated with carveol (B), citronellol (C) and geraniol (D). A: SEM of untreated E. coli strains; B: E. coli strains treated with 4× MIC of carveol; C: E. coli strains treated with 4× MIC of citronellol; D: E. coli strains treated with 4× MIC of geraniol.

Figure 3

Scanning Electron Microscopy of untreated S. aureus (A) and S. aureus treated with terpineol (B). A: SEM of untreated S. aureus strains; B: S. aureus strains treated with 4× MIC of terpineol.

Figure 4

Scanning Electron Microscopy of untreated S. Typhimurium (A) and S. Typhimurium treated with eugenol (B) and terpineol (C). A: SEM of untreated S. Typhimurium strains; B: S. Typhimurium strains treated with 4× MIC of eugenol; C: S. Typhimurium strains treated with 4× MIC of terpineol.