Antimicrobial Properties and Mechanism of Action of Some Plant Extracts Against Food Pathogens and Spoilage Microorganisms

Abstract

This work aims to evaluate the antimicrobial potential of ethanolic and water extracts of roselle (Hibiscus sabdariffa), rosemary (Rosmarinus officinalis), clove (Syzygium aromaticum), and thyme (Thymus vulgaris) on some food pathogens and spoilage microorganisms. Agar well diffusion method has been used to determine the antimicrobial activities and minimum inhibitory concentrations (MIC) of different plant extracts against Gram-positive bacteria (Bacillus cereus, Staphylococcus aureus), Gram-negative bacteria (Escherichia coli, Salmonella enteritidis, Vibrio parahaemolyticus, and Pseudomonas aeruginosa), and one fungus (Candida albicans). The extracts exhibited both antibacterial and antifungal activities against tested microorganisms. Ethanolic roselle extract showed significant antibacterial activity (P < 0.05) against all tested bacterial strains, while no inhibitory effect on Candida albicans (CA) was observed. Only the ethanolic extracts of clove and thyme showed antifungal effects against CA with inhibition zones ranging from 25.2 ± 1.4 to 15.8 ± 1.2 mm, respectively. Bacillus cereus (BC) appears to be the most sensitive strain to the aqueous extract of clove with a MIC of 0.315%. To enhance our understanding of antimicrobial activity mechanism of plant extracts, the changes in internal pH (pH_int), and membrane potential were measured in Staphylococcus aureus (SA) and Escherichia coli (EC) cells after exposure to the plant extracts. The results indicated that the plant extracts significantly affected the cell membrane of Gram-positive and Gram-negative bacteria, as demonstrated by the decline in pH_int as well as cell membrane hyperpolarization. In conclusion, plant extracts are of great value as natural antimicrobials and can use safely as food preservatives.

Keywords: antimicrobial properties; internal pH (pHint); membrane potential; pathogenic microorganism; plant extract; spoilage; ultrasound-assisted extraction.

FIGURE 1

The inhibition zone (mm) of aqueous (A,B) and ethanolic extracts (C,D) of Roselle (1), Clove (2), Rosemary (3), and Thyme (4) against Escherichia coli (EC) and Staphylococcus aureus (SA), at concentration of 20% (w/v). (0) represent the negative control, 10% v/v DMSO for ethanolic extracts, and distilled water for aqueous extracts.

FIGURE 2

Effects of aqueous and ethanolic extracts of Roselle, Clove, Rosemary, and Thyme on the cytoplasmic pH (pH_in^xt) of Escherichia coli (EC) and Staphylococcus aureus (SA). Values represent the means of triplicate measurements (n = 3). Bars represent the standard deviation.

FIGURE 3

Effects of aqueous and ethanolic extracts of Roselle, Clove, Rosemary, and Thyme on the membrane potentials of Escherichia coli (EC) and Staphylococcus aureus (SA). Values represent the means of triplicate measurements (n = 3). Bars represent the standard deviation.