Pharmacological potential of the marine peptide cyclo(L-phenylalanyl-L-prolyl) against both multidrug-resistant, gram-negative Acinetobacter baumannii and gram-positive Staphylococcus aureus: structure - activity relationship, computational and experimental studies.

The marine cyclic dipeptide, cyclo(phenylalanine-proline) (cFP), exhibits promising antimicrobial and antibiofilm activities. The balanced reactivity and stability of the compound, which are conducive to multi-target interactions, were identified using the density functional theory (DFT) calculations, and PASS (Prediction of Activity Spectra for Substances) analysis predicted cFP's antimicrobial activity based on its structure-activity relationship. Antibacterial efficacy was determined by MIC and MBC with values of 200-250 µg/mL and 400-500 µg/mL, respectively, against Acinetobacter baumannii and Staphylococcus aureus. Time-kill kinetics demonstrated bacteriostatic effects at sub-MIC concentrations, and bactericidal activity at higher concentrations, with 3D growth curves suggesting dose-dependent inhibition. The cFP mildly elevates intracellular reactive oxygen species (ROS), depletes the antioxidant glutathione, and, through ROS generation, only partially attenuated by N-acetylcysteine scavenging, indicating that redox perturbation contributes to, but does not solely account for, its antimicrobial activity. The cFP demonstrated potent antibiofilm potential, achieving a 79.3% reduction in mature biofilm biomass at 100 µg/mL, with marked fragmentation observed microscopically at sub-MIC doses. Extracellular polymeric substance (EPS) production was inhibited dose-dependently, exceeding 60% suppression at the highest concentrations. Furthermore, cFP significantly reduced bacterial cell surface hydrophobicity, thereby impairing adhesion mechanisms critical for biofilm formation. Molecular docking and 100 ns MD simulations suggest that cFP can form stable interactions with virulence-associated proteins, including FabI, AceR, GyrB, and SarA, which are established antibacterial or antivirulence targets with known reference ligands such as triclosan, chlorhexidine, novobiocin, and 2-[(methylamino)methyl]phenol, respectively. The cFP exhibited strong hemocompatibility with minimal hemolytic activity (< 5%), indicating low erythrocyte membrane toxicity. Taken together, the experimental and computational findings suggest that cFP is a promising antimicrobial lead and warrant further mechanistic and translational investigation.