Antibacterial and chemical profiling of Ulva intestinalis collected from Egypt

The increasing limitations of antibiotic therapy, due to adverse effects and the alarming rise of bacterial resistance, underscore the urgent need for novel antimicrobial alternatives. This study investigated the antibacterial and antibiofilm activities of crude extracts from the green seaweed Ulva intestinalis as potential natural substitutes for conventional antibiotics. Samples were collected from the Mediterranean coast of Alexandria, Egypt, and extracted using ethanol, methanol, and acetone. The antibacterial activity against Staphylococcus aureus was assessed via the agar well diffusion method. Among the tested extracts, the ethanol extract at 200 µg/mL showed the largest inhibition zone (35.03 ± 0.04 mm), whereas the acetone extract exhibited the lowest activity (15.7 ± 0.09 mm) at the same concentration. The antibiofilm efficacy, evaluated using a 96-well microtiter plate assay, revealed that the methanolic extract inhibited biofilm formation by 91.26% at 200 µg/mL, the highest tested concentration. GC-MS analysis identified several bioactive constituents, including pentadecane, eicosane, and 2,6,10,14-tetramethylpentadecane, whereas FTIR spectra confirmed the presence of phenolic, alcoholic, aliphatic, ketonic, aromatic, and amine functional groups. Molecular docking studies demonstrated strong binding affinities of selected compounds toward key S. aureus targets (RNA polymerase, DNA gyrase, and PBP2a), suggesting potential mechanisms involving disruption of transcription, DNA replication, and β-lactam resistance. Overall, these results highlight U. intestinalis as a promising marine-derived source of antimicrobial and antibiofilm agents with potential pharmaceutical applications.