Isolation and identification of an antibacterial compound from Diplotaxis harra (Forssk.) Boiss

The emergence of the antibiotic resistance is an ongoing problem in public health, and therefore the search for new natural molecules represents an alternative to synthetic drugs. The aim of this study was to test the antibacterial activity of ten Mediterranean plants (Diplotaxis harra, Ecballium elaterium, Pergularia tomentosa, Myrtus communis, Solanum villosum, Solanum sodomaeum, Peganum harmala, Lepidium sativum, Pistacia lentiscus, and Calendula arvensis) against seven pathogenic bacteria (Staphylococcus aureus, Escherichia coli, Listeria monocytogenes, Pseudomonas aeruginosa, Klebsiella sp., Salmonella enteritidis, and Bacillus cereus) using the disk diffusion method, in order to isolate and identify the active compound(s). Dichloromethane extract of D. harra flower showed the best activity against S. aureus and L. monocytogenes (MIQ = 30 μg/disk and 15 μg/disk, respectively). This extract was submitted to a bio-guided purification using Thin Layer Chromatography (TLC)-bioautography, and an antibacterial fraction (MIQ = 2 μg/disk) was isolated. The active fraction was characterized by RP-HPLC-DAD-ESI-MSn and GC–MS. Sulforaphane, an isothiocyanate known for its anti-cancer, antioxidant, and anti-inflammatory properties, was identified as antibacterial agent in D. harra for the first time. Due to its high antibacterial activity, sulforaphane could be considered a good candidate for the selection of new natural antibacterial molecules.