The Antibacterial Activity of Date Syrup Polyphenols against S. aureus and E. coli

Abstract

Plant-derived products such as date syrup (DS) have demonstrated antibacterial activity and can inhibit bacteria through numerous different mechanisms, which may be attributed to bioactive compounds including plant-derived phenolic molecules. DS is rich in polyphenols and this study hypothesized that DS polyphenols demonstrate inherent antimicrobial activity, which cause oxidative damage. This investigation revealed that DS has a high content of total polyphenols (605 mg/100 g), and is rich in tannins (357 mg/100 g), flavonoids (40.5 mg/100 g), and flavanols (31.7 mg/100 g) that are known potent antioxidants. Furthermore, DS, and polyphenols extracted from DS, the most abundant bioactive constituent of DS are bacteriostatic to both Gram positive and Gram negative Escherichia coli and Staphylococcus aureus, respectively. It has further been shown that the extracted polyphenols independently suppress the growth of bacteria at minimum inhibitory concentration (MIC) of 30 and 20 mg/mL for E. coli and S. aureus, and have observed that DS behaves as a prooxidant by generating hydrogen peroxide that mediates bacterial growth inhibition as a result of oxidative stress. At sub-lethal MIC concentrations DS demonstrated antioxidative activity by reducing hydrogen peroxide, and at lethal concentrations DS demonstrated prooxidant activity that inhibited the growth of E. coli and S. aureus. The high sugar content naturally present in DS did not significantly contribute to this effect. These findings highlight that DS's antimicrobial activity is mediated through hydrogen peroxide generation in inducing oxidative stress in bacteria.

Keywords: Phoenix dactylifera L.; S. aureus; date syrup; polyphenol.

FIGURE 1

Comparison of the antioxidant potential of date syrup (DS) (round gray scale dot) and extracted DS polyphenol (PPDS) (black triangle) against the commercially available antioxidant butylated hydroxytuolene (BHT) (black dot). Results are expressed as mean ± SD of three independent experiments and significant difference between DS treatments and BHT is indicated as ^∗p < 0.05.

FIGURE 2

Inhibitory effects of catalase on the antibacterial action of DS. DS treatments; DS, extracted DS polyphenols (PPDS) and artificial DS sugar (Sugar) were added to (A) Escherichia coli and (B) Staphylococcus aureus cell suspensions with or without 100 U/mL catalase. After being incubated at 37°C for 4 h with shaking, cell viability was determined using surface drop count methods and expressed as viability in colony forming units (CFU). Data is mean ± SD of three independent experiments. Significant differences between treatment groups are indicated as ^∗p < 0.05.

FIGURE 3

H₂O₂ generation by different sample treatments of DS, extracted DS polyphenols (PPDS), and artificial DS sugar (Sugar) and the effect of 100 U/mL catalase on the production of hydrogen peroxide by DS. The concentration of H₂O₂ in the medium was immediately determined by the FOX method 1 h after the addition of different DS treatments to NB medium (pH7.5). Data is mean ± SD of three independent experiments. Significant differences between treatment groups are indicated as ^∗∗p < 0.01.

FIGURE 4

Evaluation of antioxidant activity of DS treatments. (A) 15 mg/mL DS, (B) 30 mg/mL DS, and (C) polyphenol DS (PPDS)(30 mg/mL). DS treatments were added to NB medium and incubated at 37°C with E. coli for 4 h with shaking, cell viability was determined using the surface drop count method and expressed as viability in CFU. Significant differences between treatment groups are indicated as ^∗p < 0.05.

FIGURE 5

Evaluation of the synergistic effect of DS and H₂O₂ on cellular viability. Significant differences between treatment groups are indicated as ^∗∗p < 0.01.