Evaluation of in vitro antibacterial activity and in vivo Caenorhabditis elegans assays of Bacillus velezensis AU1 and AU2

Alkaliphilic bacteria, including Bacillus species, represent a promising but underexplored source of bioactive compounds with potential antibacterial applications. The growing concern over antibiotic resistance highlights the need for discovering new antibacterial agents. This study aimed to evaluate the in vitro antibacterial activity of Bacillus velezensis strain AU1 and AU2 isolated from Montañeza Waterfalls and assess their in vivo efficacy using Caenorhabditis elegans as a model organism. AU1 and AU2 isolates were subjected to bacterial fermentation, followed by liquid-liquid extraction using ethyl acetate as a solvent to yield crude extracts. AU1 and AU2 extracts exhibited antibacterial activity against Gram-positive bacteria, Bacillus subtilis, and Gram-negative Escherichia coli, with zones of inhibition ranging from 12.40−15.63 mm and 8.47−14.10 mm, respectively. AU1 increased the lifespan of the worms, improved locomotion, facilitated higher reproduction rates, and was strongly preferred by the worms over E. coli OP50 control or AU2. Furthermore, worms treated with AU1 were also able to survive during Gram-positive bacterial infection, indicating that AU1 may offer superior nutritional benefits that contribute to these positive outcomes. Further investigation is warranted to isolate and characterize bioactive compounds from crude extracts as potential sources for new or novel antimicrobial drugs.