Winemaking is currently one of the most relevant agroindustrial activities in the world. Undoubtedly, grapes are an abundant fruit crop worldwide, with Vitis vinifera being the species most frequently cultivated for wine production (Pareja et al., 2015; Barba et al., 2016).

Resveratrol (3,5,4′-trihydroxystilbene) is a naturally occurring stilbenoid which has been gaining considerable attention in the medical field due to its diverse biological activities – it has been reported to exhibit antioxidant, cardioprotective, anti-diabetic, anticancer, and antiaging properties. Given that resveratrol is a phytoalexin, exhibiting an increased synthesis in response to infection by phytopathogens, there has been interest in exploring its antimicrobial activity (Chan, 2002).

Although there is still very limited work on the antibacterial activity of resveratrol, it has been shown that resveratrol exhibits antibacterial activity against several Gram-positive and Gram-negative foodborne bacteria (Chan, 2002; Tegos et al., 2002; Paulo et al., 2010; Paolillo, Carratelli and Rizzo, 2011; Alvarez, Moreira and Ponce, 2012; Alvarez, Ponce and Moreira, 2013; Kumar et al., 2012; Plumed-Ferrer et al., 2013; Augustine et al., 2014; Ferreira et al., 2014; Morán et al., 2014; Promgool, Pancharoen and Deachathai, 2014; Subramanian, Soundar and Mangoli, 2016; Duarte et al., 2015; Kim et al., 2014; Makwana et al., 2015; Ferreira and Domingues, 2016; Liu et al., 2016; Seukep et al., 2016; Silva et al., 2016; Surendran Nair et al., 2016; Klancnik et al., 2016; Lai, Chiu and Chiou, 2017; Lee and Lee, 2017; Oliveira, Domingues and Ferreira, 2017).

Based on the available literature, resveratrol has been demonstrated to exhibit different antibacterial activities against numerous strains of foodborne pathogens including Staphylococcus aureus, Bacillus cereus, Bacillus subtilis, and Listeria monocytogenes, E. coli O157:H7, Salmonella Typhimurium, Vibrio cholera, Campylobacter jejuni, Campylobacter coli, Arcobacter butzleri, and Arcobacter cryaerophilus (Ma et al., 2018).

Grape pomace is a potential source of natural antioxidant and antimicrobial agents. The phenolic compounds in grape pomace extracts exhibit antioxidant, anticancer, and antidiabetic properties (Ruberto et al., 2007; Hogan et al., 2009; Parry et al., 2011; Zhou and Raffoul, 2012; González-Centeno et al., 2013; Snopek et al., 2018), as well as antibacterial activity against E. coli, L. monocytogenes, and S. aureus (Ozkan et al., 2004; Darra et al., 2012).

More recently, the impact of the gastrointestinal digestion step on the phytochemical content in food, and on their bioactivities (mainly antioxidant capacity), have attracted special attention, which is evidenced by a great number of publications dedicated to this theme (Gumienna, Lasik and Czarnecki, 2011; Tavares et al., 2012; Correa-Betanzo et al., 2014).

The aim of this work was to demonstrate the antimicrobial properties of resveratrol and red grape pomace extract towards both Grampositive and Gramnegative bacteria as well as yeasts. The microorganisms’ sensitivity to resveratrol and red grape pomace extract were determined using the disk diffusion method, while the broth microdilution method was selected to determine the minimal inhibitory concentration (MIC).

The disk diffusion method shoved that resveratrol exhibited a better antibacterial activity against all tested Grampositive bacteria (Table 1), when compared to Gramnegative bacteria. The lowest antimicrobial activity was found against yeasts which is consistent with previous studies using similar strains (Paulo et al., 2010). The antimicrobial activity using the disc diffusion method ranged from 10.00 ±2.00 (C. albicans) to 22.67 ±2.08 (E. faecalis).

Similar results were found in case of the pomace extract. The best antimicrobial activity was found against E. faecalis while the lowest antibacterial activty was detected against C. krusei.

The high antimicrobial effect was observed in the variants with grapevine seeds against E. coli (Jakubcová et al., 2015).

In study of Chan (2002) established that resveratrol conferred the antibacterial effect. DMSO did not reduce the growth of the tested bacteria, except for a slight decrease in the case of E. faecalis at 3.3%. When present at 171 μg.mL^-1 of resveratrol in 1.7% DMSO, the growth of S. aureus was inhibited by 80 – 90%. A similar degree of inhibition was observed in case of E. faecalis and P. aeruginosa at 342 μg.mL^-1 of resveratrol in 3.3% DMSO.

In our work, resveratrol concentrations ranging from 2 to 512 μg.mL^-1 have been tested, and its MIC for all Grampositive bacteria (Staphylococcus aureus, Enterococcus faecalis and Listeria monocytogenes), Gramnegative bacteria (Escharichia coli, Pseudomonas aeroginosa and Salmonella enteritidis) and yeasts (Candida albicans, Candida krusei and Candida tropicalis) were determined. The microorganism that presented the highest sensitivity towards resveratrol was Enterococcus faecalis (MIC 64 μg.mL^-1), followed by Staphylococcus aureus and Listeria monocytogenes, which have presented with a MIC of 128 μg.mL^-1 (Table 2). Similar results with similar bacteria were obtained in the study by Paulo et al. (2010). In this work, resveratrol concentrations ranging from 3.125 to 400 μg.mL^-1 have been tested, and its MIC for all Grampositive bacteria (Bacillus cereus, Staphylococcus aureus and Enterococcus faecalis) was determined. The microorganism that presented the highest sensitivity towards resveratrol was Bacillus cereus ATCC 11778 (MIC 50 μg.mL^-1), followed by Staphylococcus aureus ATCC 25923 and Enterococcus faecalis ATCC 29212, with a MIC of 100 μg.mL^-1.

It was not possible to obtain resveratrol concentrations higher than 400 μg.mL^-1 due to its poor solubility (Jeandet et al., 2002), and therefore its MIC on Gram-negative bacteria was not determined. Consequently, the efficacy of inhibition (in terms of percentage) presented by different concentrations of resveratrol was used to evaluate its activity against these bacteria. At a concentration of 400 μg.mL^-1, the inhibition percentages observed for Escherichia coli, Salmonella typhimurium and Klebsiella pneumoniae were respectively, 81, 80 and 58%. In addition, it was not possible to determine the MBCs, since the maximum tested concentration was 400 μg.mL^-1.

Kačániová et al. (2018) tested four strains of bacteria (two Gram-positive bacteria Staphylococcus aureus CCM 2461, Bacillus cereus CCM 2010; two Gram-negative bacteria Escherichia coli CCM 3988, Pseudomonas aeruginosa CCM 1959) and four yeasts strains (Candida albicans CCM 8186, Candida glabrata CCM 8270, Candida krusei CCM 8271 and Candida tropicalis CCM 8223). For the detection of the antimicrobial activity, the grape pomace extracts of white variety Pálava and red variety Dornfelder were used. Pálava pomace extracts were less efficient against the microorganisms tested and Dornfelder extracts were more active against Grampositive bacteria and yeasts. The best antimicrobial activity of pomace extract Blue Frankish grape in our study was found to be similar to resveratrol against Grampositive bacterial strains. The antibacterial activity of four grape pomace extracts was evaluated in the study of Xu et al. (2015). All extracts exhibited antibacterial activity against L. monocytogenes and S. aureus, but no antibacterial activity was detected against E. coli O157:H7 and S. typhimurium. Our results partially agree with previous studies on the antimicrobial activity of whole grapes or grape pomace extracts against both Gram-positive and Gram-negative bacteria, with the most pronounced effects against Gram-positive bacteria (Darra et al., 2012).

The present study showed that resveratrol and red pomace extract of Blue Frankish grape showed better antibacterial activity against Grampositive Staphylococcus aureus, Enterococcus faecalis and Listeria monocytogenes when compared with Gramnegative bacteria and yeasts using both methods, the disc diffusion and the minimal inhibiotn concentration.