Bioassay-Guided Investigation of Antibacterial Metabolites from Providencia rettgeri PMRU2.3 Isolated from Chaetomorpha linum (O.F. Müller) Kützing.

The escalating global crisis of antimicrobial resistance (AMR) necessitates the urgent exploration of novel bioactive compounds from untapped marine ecosystems. Seaweed-associated (epiphytic) bacteria represent a promising but understudied reservoir of chemically prolific microorganisms. This study aimed to isolate, characterize, and evaluate the pharmacological potential of bacteria associated with the green macroalga Chaetomorpha linum, collected from the Manapaadu coast, India. A total of ten bacterial isolates were recovered, with strain MG51 exhibiting the most potent antagonistic activity. Molecular identification via 16s rRNA sequencing confirmed the isolate as Providencia rettgeri strain PMRU2.3 (GenBank Accession: ON604798.1). Chemical profiling of the ethylacetate extract using Fourier Transform Infrared Spectroscopy (FT-IR) and Gas Chromatography-Mass Spectrometry (GC-MS) revealed diverse functional groups and sixteen bioactive compounds. The dominant metabolite was Propanamide, 2-hydroxy- (42.13%), a predicted proteasome inhibitor .The crude extract demonstrated significant broad-spectrum antibacterial activity, particularly against Gram-negative pathogens, showing maximum inhibition against Pseudomonas aeruginosa (79.0%) and Klebsiella pneumoniae (71.3%). Unlike typical clinical isolates, strain PMRU2.3 was susceptible to standard antibiotics, suggesting it is a non-virulent environmental lineage. Biosafety assessment using the Brine Shrimp Lethality Assay indicated significant bioactivity with an LC50 of 639.28 µg/L. This study reports the first evidence of antibacterial metabolite production by P. rettgeri isolated from the phycosphere of C. linum. The presence of potent bioactive compounds suggests that this marine epiphyte is a viable candidate for bioprospecting novel pharmaceutical agents to combat multidrug-resistant infections.