The Effect of Thyme Essential Oil on Escherichia coli

Abstract

Research on the effectiveness of essential oils extracted from plants is limited. However, some studies have shown that essential oils have health benefits, including its antimicrobial property. The objective of this study is to analyze the effect of thyme essential oil under standardized conditions on Escherichia coli. Escherichia coli (ATCC 25404™) inoculum was prepared by growing the bacteria in Luria-Bertani (LB) broth for 24 hours at 37°C, yielding 8.7 x10 CFU/ml. One hundred microliters of the starting bacterial culture were spread onto LB agar plates and left to dry for 15 minutes. Treatments were formed by creating thyme essential oil solutions (EO) of 0, 2.5, 10, 25% using 70% ethyl alcohol as the diluent. Each EO was vortexed and filtered using a 0.2 microliter sterile syringe filter. Twenty microliters of each EO were pipetted into a sterile paper disc (weight ranged of 0.0086-0.009 g). Three paper discs of each treatment containing the EO were placed on standardized locations on each inoculated plate. Each treatment was composed of 9 inoculated plates. The plates were then incubated at 37C for 24 hours. Additionally, three manufactured standard antibiotic paper discs of 5 mcg Ciprofloxacin, 10 mcg Ampicillin, and 10 mcg Streptomycin were placed on separate plates as controls. The zone of inhibition created was measured using an image processing program, Image J. All data were analyzed using the general linear model procedure for analysis of variance (ANOVA). The antimicrobial property of the EO observed as the zone of inhibition increased as the concentration of the EO increased (R= 0.94, P < 0.0001). EO at 25% showed the greatest antimicrobial effect against Escherichia coli compared 10, 2.5, and 0% (33.6, 21.0, 11.5, and 0.0 mm, respectively; P < 0.0001). Ciprofloxacin antibiotic control had the highest zone of inhibition compared to all the other treatments (31.32 vs. 19.8 mm; P < 0.0001). The 25% and 10% EO showed greater effectiveness against Escherichia coli with zone of inhibition greater than ampicillin and streptomycin (33.6, 21.0 vs. 20.0, 13.0 mm respectively; P < 0.0001). However, the 2.5% EO was not as effective as Streptomycin (11.5 vs. 13.0 mm; P< 0.0001). 10% EO had zone of inhibition similar to ampicillin (21.0 and 20.0 mm respectively, P< 0.0001). In conclusion, thyme EO showed increasing antimicrobial properties against Escherichia coli as the concentration of oil increased. It proved to be more effective than two of the antibiotic controls at concentration as low as 10% EO solutions. Further evaluation of the effects of thyme EO is needed to further our knowledge of its benefit as a natural alternative.