Assessment of the Antimicrobial Activity of Olive Leaf Extract Against Foodborne Bacterial Pathogens

Yanhong Liu, Lindsay C McKeever, Nasir S A Malik, Yanhong Liu, Lindsay C McKeever, Nasir S A Malik

Abstract

Olive leaf extract (OLE) has been used traditionally as a herbal supplement since it contains polyphenolic compounds with beneficial properties ranging from increasing energy levels, lowering blood pressure, and supporting the cardiovascular and immune systems. In addition to the beneficial effects on human health, OLE also has antimicrobial properties. The aim of this work was to investigate the antimicrobial effect of OLE against major foodborne pathogens, including Listeria monocytogenes, Escherichia coli O157:H7, and Salmonella Enteritidis. Our results demonstrated that at a concentration of 62.5 mg/ml, OLE almost completely inhibited the growth of these three pathogens. In addition, OLE also reduced cell motility in L. monocytogenes, which correlated with the absence of flagella as shown by scanning electron microscopy. Moreover, OLE inhibited biofilm formation in L. monocytogenes and S. Enteritidis. Taken together, OLE, as a natural product, has the potential to be used as an antimicrobial to control foodborne pathogens.

Keywords: L. monocytogenes; bacterial growth inhibition; biofilm formation; natural antimicrobials; olive leaf extract.

Figures

FIGURE 1
FIGURE 1
High performance liquid chromatography (HPLC) chromatogram of polyphenols from olive leaf extract (OLE) using Waters Symmetry C18 (5 μm particle) column (3.9 mm × 150 mm) maintained at 35°C. The flow rate was 1 ml/min and the absorbance changes were monitored at 280 nm.
FIGURE 2
FIGURE 2
Motility of Listeria monocytogenes F2365 cells in varying concentrations of commercial OLE.L. monocytogenes F2365 strain was stabbed into BHI soft agar (0.3%) and incubated at room temperature for 48 h. The diameters of the bacterial growth rings were measured. Data presented are the averages of nine measurements with standard deviations.
FIGURE 3
FIGURE 3
Scanning Electron Microscopy pictures (magnification 50000×) of L. monocytogenes cells in different concentrations of OLE. (A)L. monocytogenes in 0.3% BHI agar plate. (B)L. monocytogenes in 7.8 mg/ml (1/8XMIC) OLE. (C)L. monocytogenes in 31.3 mg/ml (1/2XMIC) OLE. (D)L. monocytogenes in 62.5 mg/ml (MIC) OLE.
FIGURE 4
FIGURE 4
Biofilm formation of L. monocytogenes at different concentrations (1/8X and 1/4X MICs) of OLE. Assays were performed in PVC microtiter plates by incubating L. monocytogenes with different concentrations of OLE in Modified Welshimer’s Broth (MWB) media at 30°C for 48 h. Values are means and standard deviations of eight replicates.
FIGURE 5
FIGURE 5
Biofilm formation of Salmonella Enteritidis at different concentrations (1/8X and 1/4X MICs) of OLE. The biofilm growth was assessed by crystal violet staining and quantified at 595 nm using a spectrophotometer. Values are means and standard deviations of eight replicates.

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