In Portugal, wine production is one of the most important and relevant socio-economic activities. Winery by-products are rich in phenolic compounds which exhibit a wide range of physiological properties. Therefore, the aim of this study was to extract phenolic compounds from grape skin, seeds, stems, shoats and leaves from the Tinto Cão variety and evaluate their antioxidant activity and antibacterial properties against antibiotic resistant bacterial strains.

The polyphenolic constituents were extracted using water/ethanol (20:80) mixture. The extracts were dissolved in dimethyl sulfoxide (DMSO) to a final concentration of 100 μg/mL. Antimicrobial susceptibility assay was performed using Kirby-Bauer disc diffusion method and was tested against ten different bacteria: Listeria, Pseudomonas aeruginosa, Enterecoccus faecium, Klebsiella pneumoniae, Staphylococcus aureus, Salmonella enteriditis, Enterococcus faecalis, Bacillus cereus, Staphylococcus epidermidis e Escherichia coli. The antioxidant activity of the extracts was determined by DPPH, FRAP and CuPRAC methods.

The higher antioxidant activity was found in the seed extracts, followed by the leaves extracts. The shoats extracts showed the highest antimicrobial properties with minimum inhibitory concentration (MIC) of 25mg/mL, except for B. Cereus, followed by the stems extracts. None of the extracts showed antibacterial activity against E. faecium, S. enteriditis, E.faecalis and E.coli.

These results show that, natural products, such as polyphenols, may represent a promising mean to fight microbial resistance. However, they were not effective against all bacteria which shows that polyphenols, alone, might not substitute antibiotics. Thus, the phenolic compounds may be used in food and pharmaceutical industries as natural food preservers and antibiotic adjuvants.