Characterization of an Antibacterial Compound from the Mangrove Bacterium Bacillus velezensis ES8024 and the Impacts of Anthropogenic and Climatic Factors on Its Production

Abstract The unique characteristics of mangrove sediments facilitate the ability of mangrove bacteria to produce bioactive compounds. The present study focused on the antibiotic-producing bacteria of the Bhitarkanika mangrove ecosystem, India and uncovered the nature of a bioactive compound present in the mangrove bacterium Bacillus velezensis ES8024. The impact of such external stressors as pH (pCO2), salinity, and lead (Pb) concentration on antimicrobial activities of B. velezensis ES8024 was also elucidated. The bacterium produced antimicrobial substances and was active against Aeromonas hydrophila ATCC 35654. Optimization revealed maximum production of the compound at 48 h of incubation. ATR-FTIR and 1H NMR data suggested that the extracted antibacterial compound was of a lipopeptide nature. UHPLC-TOFMS data confirmed the presence of four isoforms of the antimicrobial lipopeptide surfactin, e.g. C13 (1030.83 m/z), C14 (1044.85 m/z), and C15 (1058.87 and 1059.87 m/z) as the principle antibiotic components. The presence of two surfactin synthase genes, srfAA and srfAB, was further verified within the bacterium followed by homology modeling of SrfAA and SrfAB. Under different modifiers, lowest bacterial growth and highest intracellular reactive oxygen species (ROS) production was observed at pH 4, 4% NaCl, and 1600 ppm Pb. The expression patterns of the srfAA and srfAB genes also followed a similar trend under the stressors. Prominent structural and functional alterations of the antibacterial surfactin molecules under different stress conditions were documented using ATR-FTIR, 1H NMR and well diffusion assay. The findings of the study revealed the efficiency of the antibacterial surfactin synthesized from a mangrove bacterium B. velezensis ES8024 against Aeromonas hydrophila ATCC 35654 and the effectiveness of the compound with respect to future climate change and heavy metal contaminated environment.