Identification and characterization of endophytic bacteria for antibacterial activity against multi drug resistant pathogens

The escalating threat of antibiotic resistance demands innovative approaches to antimicrobial discovery. This study investigated endophytic bacteria isolated from specific tissues of four medicinal plants, including Calotropis gigantea leaves and Acacia nilotica stems, as potential sources of novel antibacterial agents. Among 108 isolated endophytes, two strains demonstrated exceptional activity against multidrug-resistant (MDR) pathogens: CGL8 (isolated from C. gigantea leaves) showed potent inhibition against Salmonella typhi, Acinetobacter baumannii and carbapenem-resistant Enterobacteriaceae while ANS5 (derived from A. nilotica stems) exhibited strong efficacy against Acinetobacter baumannii and methicillin-resistant Staphylococcus aureus in disc diffusion assays. Phylogenetic analysis of 16S rRNA sequences confirmed strain ANS5 as Escherichia fergusonii and CGL8 as Citrobacter koseri, revealing distinct clades with robust bootstrap support ( greater than 90%). Both strains exhibited unexpected antimicrobial activity against MDR pathogens, suggesting potential novel mechanisms. Further genomic exploration is warranted to identify biosynthetic gene clusters underlying their inhibitory properties. Bioactive compounds from CGL8 and ANS5 were initially separated using TLC, and the antimicrobial fraction was further analyzed by LC-MS, confirming the presence of promising antibiotic candidates. The tissue-specific isolation approach yielded these active strains, with C. gigantea leaf-derived CGL8 and A. nilotica stem-originated ANS5 representing particularly valuable resources. This study highlights C. gigantea leaves and A. nilotica stems as rich sources of antimicrobial endophytes, demonstrating tissue-specific screening as a strategic approach to discover novel antibiotics against resistant pathogens