Seaweed-associated heterotrophic bacteria: are they future novel sources of antimicrobial agents against drug-resistant pathogens?

Emergence of multidrug-resistant microorganisms and requirements for novel antimicrobial compounds necessitate exploring newer habitats to develop potential bioactive leads. Culture-contingent analysis of heterotrophic bacterial flora from the seaweeds led to the isolation of bioactive strains possessing potential antibacterial properties against wide-ranging clinical pathogens viz., methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Enterococcus faecalis (VREfs). Seven of the most active strains belonging to the phylum Firmicutes isolated from a brown seaweed (Phaeophyceae) Sargassum wightii exhibited spot-over-lawn assay guided inhibition zone of larger than 30 mm. Integrated phenotypic and genotypic studies have led to the characterization of the seaweed-associated bacteria particularly belonging to the phylum Firmicutes. The organic extracts of the studied bacteria exhibited promising antibacterial properties against MRSA and VREfs with minimum inhibitory concentration ranging between 6.25 and 12.50 μg/mL. Time-kill kinetic profiles of those bacteria displayed rapid bactericidal activity against both E. coli and MRSA, showing a ≥ 3log reduction in viable cell count than the initial. Among the studied bioactive Bacillus spp, B. tequilensis MTCC13043 and B. altiitudinis MTCC13046 were found to possess functional polyketide synthase (pks) gene (MW027664 and MW027660) that could be amplified. The outcome of amplified genes encrypting for polyketide synthase in conjunction with antibacterial activities unveiled the broad-spectrum antimicrobial activities of the marine heterotrophic Firmicutes, which could be further used against the emergent problem of antibiotic-resistant bacterial pathogens.