Effect of carvacrol-loaded nanoemulsions on a bioluminescent strain of Escherichia coli O157:H7
Nanoemulsions have been shown to be effective delivery vehicles for poorly water soluble natural antimicrobials. In this study the antimicrobial activity of carvacrol (5-isopropyl-2-methylphenol) nanoemulsions against a bioluminescent Escherichia coli O157:H7 was investigated using light emission as an indicator of cell viability. Different emulsifiers (Ultralec Lecithin and Tween 20), oils (Palm stearin and Coconut oil) and various carvacrol concentrations (0, 1, 2 and 2.5%) were evaluated. Bioluminescence was monitored in situ using a Hamamatsu (photo multiplier tube) sensor module integrated with a Programmable Logic Controller interfaced with a PC for data acquisition. Bioluminescence decreased rapidly with the addition of emulsions containing increasing concentrations of carvacrol (250ppm-1000ppm). However when cells were assayed for viability, plate counts showed there was not a correlation of bioluminescence to cell inactivation. Bioluminescence was able to recover after the removal of carvacrol from the surrounding media. The same bioluminescent pattern was also observed with sub lethal doses of the oxidative uncoupler 2,4-dinitrophenol and the supplementary addition of the luciferase reaction substrate recovered the light emission in presence of carvacrol. These results suggest that carvacrol and 2,4-DNP uncouple oxidative phosphorylation reducing the ATP available for the biosynthesis of the aldehyde substrate. Previous reports suggested the mechanism of inactivation of carvacrol was membrane damage resulting in loss of cellular contents and viability. However the results of this work suggest that the mechanism of carvacrol inactivation is due to the uncoupling of oxidative phosphorylation.
San martin-Gonzalez, Purdue University.
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