Resumo

The dramatic reduction in the development of new antibiotics, and the gradual emergence of multiresistant bacteria, requires the search for new compounds able to restore or enhance the antibacterial activity of antibiotics currently used in clinical practice. A rich source of phenolic compounds with a broad-spectrum antibacterial activity is the grape pomace, a residue of the wine industry, whose production in Chile reached 93.000 tons in 2010. The aim of this study was to evaluate synergism between extracts from grape pomace with different antibiotics against S. aureus and the identification of compounds responsible of the synergic effect. To do this, we obtained methanol extract of grape pomace, sequentially fractionated with chloroform and ethyl acetate. We determined the minimum inhibitory concentration (MIC) of the fractions and antibiotics against S. aureus by the microdilution method. The interactions between fraction/antibiotics and fraction/phenolic acids, previously identified by liquid chromatography coupled with mass detector (LC-MS), were evaluated by checkerboard assays. The results showed that both fractions in combination with beta-lactams, quinolones, chloramphenicol or tetracycline, have an important synergistic effect on S. aureus with fractional inhibitory concentrations (FIC) which varied in a range of 0.05 - 0.37 (MIC decreased 7.8 – 28 times). Different phenolic acids were identified by LC-MS analysis, including gallic, vanillic and caffeic acid, which showed a synergistic effect (FIC: 0.05 to 0.09; MIC decreased 40 - 50 times). The combination of grape pomace extracts with different classes of antibiotics showed a significant synergistic effect on isolates of S. aureus, suggesting that compounds present in the extracts have different mechanisms of action. The identified phenolic acids participate in these synergistic interactions reducing significantly the MIC of quinolones, tetracycline and chloramphenicol, antibiotics identified as substrates of efflux pumps, which possibly are present in two S. aureus isolates used in this study. This suggests that the mixture of compounds and/or the pure compounds present in the extracts possess the ability to inhibit the efflux pumps. Acknowledgments Studentship of Doctoral Program CONICYT, DICYT-USACH.