The present work further validates the findings of antimicrobial potential of polyphenols-rich extracts against Gram-positive bacteria, both standard and clinical isolates. We have demonstrated that extracts from pistachios were partially active against clinical strains of Staphylococcus
spp., some of which were multi-drug resistant. Pathogenicity of S. aureus
is attributed to a wide range of virulence factors, including extracellular protease, lipase, and superoxide dismutase. The increased prevalence of multidrug resistance S. aureus
poses a serious risk to worldwide public health, and novel treatment strategies are needed to address this concern. Over the last decade, MRSA strains have become one of the main causes of mortality amongst hospital-acquired infectious diseases [19
]. S. aureus
6538P was the most sensitive strain, with complete inhibition achieved with a concentration of 125 and 31.25 μg mL−1
of NPRE and RPRE, respectively. Overall, we have found RPRE was more effective than NPRE. This trend, which cannot be explained by the total amount of polyphenols present in the two extracts, or by their antioxidant potential, could be attributed to the qualitative composition of the two extracts (Figure 1
). In agreement with our previous investigation [15
], the concentration of gallic acid and eryodictiol-7-O-glucoside was higher in RP than NP. Lee et al. [21
] have recently reported on the antibacterial activity of a multifunctional nanoparticle containing gallic acid against methicillin resistant S. aureus
strains: the bactericidal activity of functionalized nanoparticles containing gallic acid was increased compared to the non-functionalized nanoparticles, with high selectivity for MRSA strains. Extracts from phenolic blueberry and blackberry pomace rich in phenolic acids, mainly protocathecuic, cumaric, vanillic, caffeic, and gallic acids, were able to inhibit the growth of vegetative MRSA in vitro and MRSA biofilm formation on plastic surface [22
]. A recent investigation [23
] reported on the protective role played by eriodyctiol against S. aureus
induced lung cell injury by inhibiting alpha-hemolysin expression.
The differences between the two extracts, possibly due to the roasting procedure, may affect their biological activity. It is known that each class of polyphenols is characterized by an activity closely related to its chemical structure, due mainly to hydroxyl groups linked to phenolic structures and their degree of glycosylation [24
]. Amongst flavanols, catechin was nearly 3 times higher in RPRE compared with NPRE, whereas hydroxybenzoic acids and flavanones were significantly higher in roasted pistachios (Figure 1
). From our preliminary investigations, it was observed that NPRE was richer in polyphenols and consequently had greater antioxidant activity compared to RPRE. The total phenols values were higher than those previously observed, which substantially vary with regard to the pistachio kernels, ranging from 165 ± 8.00 to 347 ± 34.00 mg GAE/100 g [17
]. Natural raw pistachios were found more active than roasted salted pistachios in our recent in vitro and in vivo studies [26
Galloyl flavan-3-ols such as (-)-epicatechin gallate and catechins are effective against MRSA strains [28
], whereas (-)-epicatechin gallate sensitises MRSA strains to β-lactam antibiotics [29
]. The use of pistachio polyphenols in combination with traditional or antibiotics could identify new mechanisms of synergism and modulate properties of antibiotic resistance. This could aim to the development of novel topical agents for the treatment of S. aureus
skin infections as well as for topical formulations.
In conclusion, the results of the present study demonstrated that polyphenols from pistachios are effective against ATCC strains of S. aureus and clinical strains of Staph. spp. Further studies are needed to establish possible synergistic effect with antibiotics in order to develop novel chemotherapic agents for the treatment of S. aureus infections.