Disruption of Escherichia coli, Listeria monocytogenes and Lactobacillus sakei cellular membranes by plant oil aromatics.

The role of membrane disruption in the bactericidal activity of the plant oil aromatic compounds eugenol, carvacrol and cinnamaldehyde was investigated using confocal laser scanning microscopy, changes in ATP levels and cell viability. In 25 mM HEPES buffer pH 7 at 20 degrees C, 10 mM eugenol or carvacrol increased uptake of propidium iodide by Escherichia coli, Listeria monocytogenes and Lactobacillus sakei over a 10-min period. The same treatments resulted in lowered viability, rapid depletion of cellular ATP and release of ATP, with the exception of Lb. sakei treated with carvacrol. Eugenol or carvacrol at 5 mM to 10 mM inhibited E. coli and L. monocytogenes motility. Lb. sakei was resistant to cinnamaldehyde. Thus, its effects were only studied on E. coli and L. monocytogenes. At 10 mM, cinnamaldehyde caused a slight but statistically significant increase in propidium iodide staining of E. coli, but had no effect on L. monocytogenes. Cinnamaldehyde treatment of E. coli at 10 mM and L. monocytogenes at 40 mM resulted in decreased cellular ATP, but there was no concomitant release of ATP. Cinnamaldehyde at 5 and 10 mM inhibited E. coli and L. monocytogenes motility. Results for eugenol and carvacrol are consistent with non-specific permeabilization of the cytoplasmic membrane. Evidence for increased membrane permeability by cinnamaldehyde is less conclusive. The release of ATP from eugenol and carvacrol-treated cells and absence of release from cinnamaldehyde-treated cells could indicate that eugenol and carvacrol possess ATPase inhibiting activity. Secondary effects would also be consistent with membrane disruption.