Isolation and Characterization of Indigenous Endophytic Bacteria from Mangrove Plants and Their Potential to Improve Seed Physiological Quality

Mangrove ecosystems are characterized by fluctuating salinity and osmotic conditions that may support diverse stress-adapted endophytic bacteria. This study aimed to isolate culturable endophytic bacteria from mangrove plants in Southeast Sulawesi, evaluate their growth responses under PEG-induced osmotic stress, screen for presumptive plant growth-promoting traits, and assess their effects on the physiological quality of local tomato seeds. Mangrove samples were collected from 4 locations, and bacterial isolates were obtained using surface sterilization and cultivation on tryptic soy agar. Bacterial growth responses were evaluated in Nutrient Broth supplemented with PEG 6000 at calculated target osmotic potentials ranging from −1.00 to −2.00 MPa. Preliminary plant growth-promoting characteristics were examined using the Salkowski reaction, growth on nitrogen-free Burk's medium, and phosphate solubilization on Pikovskaya medium. A total of 43 bacterial isolates were obtained, with the highest number recovered from Lalowaru Village. Sixteen isolates were selected for further characterization at the target osmotic potential of −2.00 MPa, among which LW2B2 exhibited the highest reported optical density (OD570 = 0.709). Qualitative screening identified LH2B2, LH3B1, LH3B3, LW2B3, and TG3B2 as isolates exhibiting strongly positive responses in all three preliminary assays. In the seed bioassay, LH3B3 and TG3B2 showed favorable responses across several germination and seedling growth parameters. These findings identify promising mangrove-derived endophytic bacterial isolates for further investigation.