Antibacterial and antioxidant properties of phenolic-rich extracts from apple (Malus domestica cv. Gala)

Phytochemicals such as phenolic compounds have antimicrobial properties and are present in apples. In the present work, the antibacterial activity of apple extracts was evaluated against Escherichia coli, Listeria monocytogenes, Salmonella Typhimurium and Staphylococcus aureus. Initially, acetone-ethanol extracts of the Gala apple were purified and fractionated by solid phase extraction to obtain an unfractionated phenolic extract (UPE) and four phenolic fractions (PFs). The identification of phenolic compounds was performed by liquid chromatography-mass spectrometry. The antibacterial activity was determined through the disk diffusion method using the minimum inhibitory concentration and the minimum bactericidal concentration. The antioxidant activity was determined by 2,2-diphenyl-1-picrylhydrazyl, and the total phenolic content was estimated using the Folin-Ciocalteau reagent. The UPE and PF contained flavonoid derivatives including quercetin, rutin, epicatechin, malic acid, chlorogenic acid and phloretin. L. monocytogenes was susceptible to the UPE and to all PFs (except PF III), resulting in inhibition zones with diameters ranging from 3.75 mm to 14.68 mm. S. aureus was susceptible to nearly all PFs, except PF I and III. PF III effectively inhibited the growth of Gram-negative bacteria, which could be attributed to its prominent antioxidant activity (763.3 μg/mg fraction) and phenolic content (459.3 μg/mg fraction). However, PF IV produced inhibition zones that were similar in size but with higher antioxidant activity (1226.8 μg/mg) and phenolic content (620.6 μg/mg). All the fractions and the UPE exhibited high phenolic content and antioxidant activity. PF I, PF II, PF IV, and UPE showed strong activity against L. monocytogenes. Antibacterial activity was observed in all fractions, with fraction IV and the UPE having the broadest spectrum of action, as indicated by their ability to inhibit all Gram-positive bacteria tested. Therefore, the phenolic compounds detected in apples have the potential to be used as natural antibacterial agents and/or antioxidants in the food and pharmaceutical industry.