Exploring the Antibacterial and Antibiofilm Efficacy of Psammogeton biternatum Edgew and Identification of a Novel Quinoline Alkaloid using X-ray Crystallography

The prevalence of resistance to harmful human pathogens is steadily rising, emphasizing the urgent need to identify novel antimicrobial compounds. For this purpose, plants stand out as a significant source of bioactives worthy of exploration. Among these, alkaloids, a vast and structurally diverse category of plant secondary metabolites, have emerged as a foundation for crucial antibacterial medications such as metronidazole and the quinolones. In the current work, the crude methanol leaf extract of Psammogeton biternatum Edgew collected from District Bannu, Pakistan, was subjected to TLC (indirect) bioautography and X-ray crystallography for the isolation of potential antibacterial agents. From the crude extract, a novel quinoline alkaloid called quinoline dione ((3R,3aS,5aR)-3,5a,9-trimethyl-3a,4,5,5a-tetrahydro-2H-isoxazolo[2,3-a] quinoline-2,8(3H)-dione (C₁₄H₁₇NO₃)) was isolated. The crystal information (M = 247.296 g/mol) is as follows: orthorhombic, P2₁2₁2₁, a = 7.7339(14) Å, b = 10.7254(19) Å, c = 15.730(2) Å, V = 1304.8(4) ų, Z = 4, T = 296 K, μ(Mo Kα) = 0.088 mm^-1, ρ _calc = 1.259 g/cm³, 13928 reflections measured (5.86° ≤ 2Θ ≤ 51.98°), 2478 unique (R _int = 0.1613, R _σ = 0.1335). The final R ₁ was 0.1098 (I ≥ 2u(I)), and wR ₂ was 0.2183. The antibacterial activity for both crude extract of leaves and quinoline dione was determined by a well diffusion method. The quinoline dione alkaloid demonstrated excellent inhibition zones against methicillin-resistant Staphylococcus aureus (18 mm), Bacillus subtills (17 mm), Escherichia coli (20 mm), and Pseudomonas aeruginosa (23 mm) compared to the crude extract. The antibiofilm potential was recorded against Pseudomonas aeruginosa by the 96-well microtiter plate method. A dose-dependent biofilm inhibition response was recorded, which increased with the increase in concentration. Moreover, quinoline dione showed a greater antibiofilm effect as compared to the crude extract, which may be linked to the presence of a particular active functional group positioned on the compound isolated in its pure form. Through in silico studies, i.e., molecular docking, quinoline dione shows strong binding energies with the LasR transcriptional regulator (6MVN) at -9.3 and LasR transcriptional activator (3IX4) at -9.2 kcal/mol, as well as moderate affinities with other targets such as AHL synthase LasI (PDB ID 1RO5) and OprM channel (PDB ID 3D5K), indicating its potential as a quorum sensing inhibitor. Thus, the antibacterial and antibiofilm potential of quinoline dione was confirmed.