Chemical Profiling and Antibacterial, Anti-Biofilm, and Antioxidant Activities of Endophytic <i>Serratia marcescens</i> AI-N-1 from <i>Azadirachta indica</i>

The rising need for new antibiotics and antioxidants highlights endophytic bacteria as promising sources of bioactive compounds. Medicinal plants such as <i>Azadirachta indica</i> harbour diverse endophytes, yet their potential in southwest Nigeria remains largely underexplored. This study investigated the antimicrobial, biofilm inhibitory, and antioxidant activities of bioactive compounds produced by the bacterial endophyte <i>Serratia marcescens</i> AI-N-1, isolated from <i>A. indica</i>. Crude extracts of <i>S. marcescens</i> showed strong antimicrobial activity against <i>Bacillus subtilis</i> (79.79% inhibition) and <i>Salmonella typhi</i> (77.04% inhibition) at 5 mg/ml. In addition, most extracts also displayed potent biofilm inhibition (>80%) against both pathogens, comparable to the positive control baicalein (<i>P</i> < 0.05). Antioxidant assays revealed high radical scavenging activity, with the supernatant extract obtained after 2 days of culture exhibiting the strongest effect (DPPH: 86.61% at 0.1 mg/ml; ABTS: 99.64% at 0.1 mg/ml). Online HPLC-ABTS⁺ analysis identified serranticin as a major contributor to these antioxidant effects. HR-MS/MS profiling further revealed prodigiosin, serratamolides, and serranticin, along with putative novel lipopeptides and other metabolites, as key bioactive compounds. To our knowledge, this is the first report of a Serratia endophyte from <i>A. indica</i> in southwest Nigeria with combined antimicrobial, antibiofilm, and antioxidant activities, as well as the discovery of putative new lipopeptides. These findings highlight endophytic bacteria from Nigerian medicinal plants as promising sources of novel antimicrobial and antioxidant agents for pharmaceutical development.