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.
Highlights
Essential oils consist of a complex mixture of compounds, usually from 20 to 60, at different concentrations [1]
The objective of the present study was to investigate antibacterial and bactericidal activities of terpenes frequently reported in the secondary metabolism of plants, as well as the time of death of bacteria caused by these terpenes
The major constituents essential oils may upevaluate to 85%, the while other constituents of essential oils leave a gap in theThe literature since they report difficulty in assigning activity to major are present in trace amounts great majority of studies that evaluate the antimicrobial activity compounds or synergism between compounds, in this way it is possible that compounds in smaller of essential oils leave a gap in the literature since they report difficulty in assigning activity to major amounts alsoorcontribute tobetween the activity
Summary
Essential oils consist of a complex mixture of compounds, usually from 20 to 60, at different concentrations [1]. The main constituents of essential oils, are derived from the isoprenoid pathway, and are produced and secreted from specialized plant tissues [2]. They are composed of isoprene units (C5 ), which is the basis for their classification, i.e., two isoprene units form monoterpenes (C10 ), three units form sesquiterpenes (C15 ), four units form diterpenes (C20 ), six units form triterpenes (C30 ) and eight units form carotenoids (C40 ) [3]. 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. The ultrastructure of control and death bacteria were evaluated by scanning electron microscopy
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