Antibacterial activity of marine macroalgae-associated Bacillus spp. from the Bay of Bengal coast of India.

The marine macroalgal holobiont evolved as a functional unit by establishing a mutualistic relationship between the host and its associated microbiomes, enabling both partners to achieve ecological success. In the present study, we evaluated the antibacterial activity of marine macroalgae-associated endophytic Bacillus spp. based on microbiological assay and profiling of non-targeted extracellular metabolites using liquid chromatography/electrospray ionization-mass spectrometry (LC/ESI-MS) analysis. The antibacterial activity of 76 bacterial endophytes isolated from 32 macroalgal samples of nine genera, including green, red, and brown algae, was evaluated in this study by employing a spot-on-overlay assay against four bacterial strains, i.e., Escherichia coli MTCC9537, Klebsiella pneumoniae MTCC109, Pseudomonas aeruginosa MTCC741, and Staphylococcus aureus MTCC7443. Seventeen strains of five different Bacillus species (i.e., B. aerius, B. pumilus, B. safensis, B. stercoris, B. stratosphericus) showed antibacterial activity at least against one bacterial pathogen. Out of 17 antibacterial positive isolates, 13, 9, 5, and 3 isolates showed the zone of inhibition against E. coli MTCC9537 (28.13 ± 7.82 mm), K. pneumoniae MTCC109 (29.2 ± 5.38 mm), P. aeruginosa MTCC741 (28.0 ± 6.71 mm), and S. aureus MTCC7443 (26.21 ± 10.88 mm), respectively. The LC/ESI-MS analysis of ethyl acetate crude extracts of extracellular metabolites secreted by three Bacillus species (i.e., B. pumilus RS45_EN1, B. stratosphericus RK15_EN4, and B. aerius RS70_EN1) detected four different antibacterial compounds, including 2-heptyl-4-hydroxyquinoline N-oxide, 4-hydroxybenzoic acid, dibutyl phthalate, and myristoleic acid. Our study suggests that macroalgae-associated Bacillus spp. could be a potential source for antibacterial compounds.