Identification of punicalagin as the bioactive compound behind the antimicrobial activity of pomegranate (Punica granatum L.) peels

Extracts from 'Zhéri' and 'Hicaznar' varieties of pomegranate, Punica granatum L., were obtained by subjecting powdered peels to extraction using water, water/ethanol (1:1; v/v), ethanol, acetone and heptane. Using the agar diffusion assay, extracts with water and/or ethanol were shown to display significant antimicrobial activity with diameters of inhibition zones up to 20 mm. Ethanolic extracts, which were the most active, were fractionated using SPE, HPLC and UHPLC, and the active compounds they contain were identified by mass spectrometry. Punicalagin, under its α and β anomeric forms, was identified as the antibacterial compound in pomegranate peel extracts. Both forms were active with MIC values between 0.3 and 1.2 µg.ml^-1, and they easily converted from one to the other with an α/β equilibrium ratio of 3/7. Their spectrum of activity targeted 10 out of 13 Gram positive and two out of three Gram negative bacteria as well as a yeast strain.