In Vitro Antibacterial Properties of Pistachio (Pistacia Vera L.) Rosy Hull Phenolic Extracts

Introduction: Plants have a wide range of secondary metabolites with antimicrobial activity such as polyphenols, quinines, flavonoids, and alkaloids, with most of them acting as a plant defense against pathogens. This study aimed to evaluate the in vitro activity of pistachio rosy hull extracts against Gram-positive (Bacillus cereus, Listeria monocytogenes, Staphylococcus aureus, and Streptococcus pyogenes), Gram-negative (Escherichia coli, Klebsiella pneumonia, and Pseudomonas aeruginosa), and clinical isolates including methicillin-resistant S. aureus (MRSA) bacterial strains. Materials and Methods: The antibacterial activity of extracts was determined using disc diffusion and microdilution broth methods. The synergistic effects and susceptibility pattern of clinical isolates were determined using antibiotic discs. Results: All extracts of rosy hull showed a significant bacteriostatic and bactericidal effect on S. aureus. However, the aqueous extract was the most active one against MRSA strains. The MICs of the aqueous and methanolic hull extracts against S. aureus were 0.78 and 1.56 mg/mL, respectively. The MICs of the aqueous and methanolic hull extracts against MRSA strains were 3.12 and 6.25 mg/mL, respectively. Furthermore, in many cases, the extracts enhanced the action of antibiotics against Gram-positive bacteria including S. aureus and Listeria monocytogenes. Conclusions: The antibacterial activity of pistachio rosy hull was reported for the first time in this study. The results suggested that pistachio rosy hull is a rich source to be used to combat problems caused by MRSA in hospitals as well as S. aureus skin infections.