Inhibition of membrane bound ATPases of Escherichia coli and Listeria monocytogenes by plant oil aromatics.

Previous studies have reported that the mechanism of bactericidal action of the plant oil aromatics, eugenol, carvacrol and cinnamaldehyde involves inhibition of adenosine triphosphate generation and membrane disruption. In this study the capacity of the aromatics to inhibit the membrane bound ATPase activity of Escherichia coli and Listeria monocytogenes was investigated by experiments on isolated membranes. Inhibition of the ATPase activity of E. coli membranes was observed with 5 mM or 10 mM eugenol or carvacrol. Progressively greater inhibition by cinnamaldehyde was observed as concentration increased from 0.1 to 10 mM. L. monocytogenes ATPase activity was significantly inhibited by eugenol (5 or 10 mM), carvacrol (10 mM) and cinnamaldehyde (10 mM). Lactobacillus sakei is highly resistant to cinnamaldehyde compared to E. coli and L. monocytogenes. To determine whether this resistance was related to the relative hydrophobicity of the cell surface and hence the ability of the cell to take up the aromatics, the percentage of the three organisms partitioning in dodecane was compared. No significant difference was found between the partitioning percentage of L. monocytogenes (17.2%) and L. sakei (13.8%), indicating that surface hydrophobicity does not explain the differing sensitivity to cinnamaldehyde of these two organism. The percent partitioning of E. coli was significantly greater than both other organisms (23.3%) and may explain the greater sensitivity of E. coli to all three aromatics.

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.

Evaluation of antilisterial action of cilantro oil on vacuum packed ham.

Cilantro oil is an essential oil preparation extracted from the plant Coriandrum sativium. A series of experiments were conducted to evaluate the ability of cilantro oil to control the growth of Listeria monocytogenes on vacuum-packed ham. The in vitro minimal inhibitory concentration for five strains of L. monocytogenes was found to vary from 0.074% to 0.018% depending on strain. Cilantro oil treatments were then tested on ham disks inoculated with a cocktail of the five L. monocytogenes strains. The treatments studied were 0.1%, 0.5%, and 6% cilantro oil diluted in sterile canola oil or incorporated into a gelatin gel in which lecithin was used to enhance incorporation of the cilantro oil. Gelatin gel treatments were also conducted with 1.4% monolaurin with or without 6% cilantro oil to determine if an interaction between the antimicrobials could increase inhibition of L. monocytogenes. Treated ham was then vacuum-packed and stored at 10 degrees C for up to 4 weeks. The only cilantro oil treatment which inhibited growth of L. monocytogenes on the ham samples was 6% cilantro oil gel. Samples receiving this treatment had populations of L. monocytogenes 1.3 log CFU/ml lower than controls at week 1 of storage, though there was no difference between treatments from week 2 onward. It appears that immobilization of the antimicrobial in a gel enhanced the effect of treatments. Cilantro oil does not appear to be a suitable agent for the control of L. monocytogenes on ham. The possible reasons for reduced effectiveness of cilantro oil against L. monocytogenes on ham are discussed.