Antimicrobial Properties of Thyme Essential Oil on Salmonella Typhimurium

Abstract

Antimicrobial properties of thyme essential oil solutions (EO) oil have been identified against foodborne bacteria, but the effectiveness has not yet been fully determined. The objective of this study is to analyze the effect of thyme essential oil under standardized conditions on Salmonella Typhimurium. The Salmonella Typhimurium (ATCC 53648™) inoculum was prepared by growing the bacteria in tryptic soy broth for 24 hours at 37°C, yielding 6.9 x10 CFU/ml. One hundred microliters of the starting bacterial culture were spread onto Mueller Hinton 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 consisted 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= 0.91, P < 0.0001). EO at 25% showed the greatest antimicrobial effect against Salmonella Typhimurium compared to 10, 2.5, and 0% (21.5, 14.8, 10.5, and 0.0 mm, respectively; P < 0.0001). There was no significant difference (P=0.20) between the zone of inhibition of 25% E0 and ampicillin. All antibiotics had some effect against Salmonella Typhimurium where ciprofloxacin had the greatest effect; its effect was even greater than the 25% EO (29.5 vs. 21.5 mm; P< 0.0001). The 25% and 10% EO was more effective against Salmonella than Streptomycin (21.5, 14.5 vs. 12.7 mm respectively; P< 0.0001). However, the 2.5% EO was not as effective as Streptomycin (10.5 vs. 12.7 mm; P< 0.0001). In conclusion, thyme EO showed increasing antimicrobial properties against Salmonella Typhimurium as the concentration of oil increased. It proved to be more effective than streptomycin antibiotic control at concentration as low as 10% EO. Further evaluation of the effects of thyme EO is needed to determine the extent of its effectiveness against additional commensal and pathogenic organisms.