A Study on Combined Effect of Monoterpenoid Essential Oils and Polyphenols from Grape Pomace on Selected Pathogenic and Probiotic Bacteria Strains

In the current study, the combined effects of essential oils (EOs) and polyphenols (PPs) were investigated as potential antibiotic alternatives. Thymol and carvacrol at a ratio of 1:1 was used as the EO due to their well-proven synergistic antimicrobial effect. The PPs were an extracted and freeze-dried product from grape pomace. The treatment solutions were prepared at the EO to PP ratios of 0:10, 3:7, 5:5, 7:3, and 10:0, at total concentrations ranging from 0 to 1000 ppm. The minimum inhibition concentrations were examined on the pathogenic bacteria strains, namely Escherichia coli O157:H7, Salmonella typhimurium, and Enterococcus cloacae, and a probiotic bacterium, Lactiplantibacillus plantarum. The mixed EOs and PPs exhibited varied levels of antibacterial effect against the tested pathogenic bacteria. The MIC of EO and the EO/PP ratio of 10:0 was the best, followed by 7:3 and 5:5 of EO/PP mixed ratios. For the probiotic bacterium, the PP and EO/PP mixed solutions exhibited promoting effects on the growth of L. plantarum at certain concentrations. The results have demonstrated that the combined EOs and PPs could maximize antibacterial activity against pathogenic bacteria while stimulating the growth of the tested probiotic bacterium. This finding will provide useful insights for developing antibiotic alternatives while minimizing the adverse effects on probiotic strains.