Antibiotic properties of seaweed-associated heterotrophic Bacilli against drug-resistant pathogens

The increasing prevalence in antibiotic resistance has driven the search for novel bioactive compounds in previously unexplored marine habitats. Seaweed-associated symbionts have emerged as significant sources of novel antibacterials against nosocomial pathogens. Herein, eighty-one bacterial isolates were obtained through culture-dependent isolation of seaweed symbionts from the southern coast of India. Bacillus velezensis SK54 represented the largest share (54 %), followed by Bacillus siamensis SK53 (33 %), Bacillus subtilis SK20 (8 %), and Bacillus filamentous SK55 (5 %). B. siamensis SK53 and B. velezensis SK54, which were separated from seaweeds Turbinaria conoides and Dictyota cervicornis, respectively, demonstrated clearance zones over 25 mm on spot-over-lawn assay against multiple clinical pathogens, including methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Staphylococcus aureus (VRSA). Selected isolates were sensitive to commercially available antibiotics, and no pathogenicity was observed. The isolates did not amplify pore-forming non-haemolytic haemolysin BL (hbl) and enterotoxin (nhe) genes, and haemolysis on blood agar confirmed their non-pathogenic nature. The bacterial extracts exhibited significant antibacterial activity against the tested pathogens, including MRSA and VRSA, with a minimum inhibitory concentration (MIC) range of 6.25–12.5 μg/mL. The 700 bp type-I polyketide synthase (pks) genes (OQ657454, OQ706631, OQ737799, and OQ834957) were amplified from the heterotrophic B. siamensis SK53 and B. velezensis SK54, with a 99 % identity in the BLAST search. The substantial antibacterial potential against drug-resistant bacteria, along with the presence of genes encoding bioactive lead molecules, suggests that these marine symbiotic bacteria, B. siamensis SK53 and B. velezensis SK54, could be effective in combating emerging antibiotic resistance.