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

TLDR

This study evaluates the antimicrobial potential of ethanolic and aqueous extracts of roselle, rosemary, clove, and thyme against food pathogens and spoilage microorganisms, and investigates their effects on bacterial internal pH and membrane potential. The authors used agar well diffusion to assess antimicrobial activity and MIC of the extracts against Gram‑positive, Gram‑negative bacteria and Candida albicans, and measured changes in internal pH and membrane potential in Staphylococcus aureus and Escherichia coli. The extracts showed antibacterial and antifungal effects; ethanolic roselle inhibited all tested bacteria, ethanolic clove and thyme inhibited Candida albicans, aqueous clove was most potent against Bacillus cereus, and all extracts disrupted bacterial membrane integrity by lowering internal pH and inducing hyperpolarization, supporting their use as safe natural food preservatives.

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 (pHint), 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 pHint as well as cell membrane hyperpolarization. In conclusion, plant extracts are of great value as natural antimicrobials and can use safely as food preservatives.