Evaluation of antimicrobial and antioxidant activities of natural phenolic compounds against foodborne pathogens and spoilage bacteria

MIC values of seven pure phenolic compounds (thymol, carvacrol, eugenol, hydroquinone, p-hydroxybenzoic acid, protocatechuic acid and gallic acid) were assessed against several strains of two Gram-positive (Staphylococcus aureus and Bacillus cereus) and two Gram-negative (Escherichia coli - including Shiga toxin-producing serogroups, STEC - and Pseudomonas fluorescens) bacteria, by using a standardized microdilution assay (ISO 20776-1:2006). Carvacrol and thymol were the most effective compounds for all genera/species studied, with the exception of S. aureus, for which hydroquinone was the most effective compound. However, we detected significant differences (p < 0.05) in the inhibition pattern of the four genera/species for both compounds (thymol and carvacrol), as well as for hydroquinone. Concerning to the less effective compounds, the four genera/species showed great differences among each other, hydroquinone being the least effective for E. coli, gallic acid for B. cereus and P. fluorescens, and eugenol for S. aureus. Gram-positive bacteria were more sensitive than Gram-negative ones for the majority of the compounds, although a general inter- and intraspecific pattern of behaviour in Gram-positive and Gram-negative bacteria was not detected. Each individual phenolic compound did not show a uniform antimicrobial effect on all strains included in each of the genera/species. Gallic acid and hydroquinone exhibited the highest antioxidant capacity and thymol and carvacrol the lowest. Gallic acid was effective in the control of S. aureus at lower concentrations than those used in the food industry as flavouring, and also showed important antioxidant ability. Therefore, this compound would be of interest in the control of S. aureus in foods. (C) 2012 Elsevier Ltd. All rights reserved.