Marine macroalga-associated heterotrophic Bacillus velezensis: a novel antimicrobial agent with siderophore mode of action against drug-resistant nosocomial pathogens.

Increased prevalence of microbial resistance and development of drug-resistant pathogens have triggered an urge among researchers to discover potential antimicrobial compounds, particularly from the marine habitat. The present study highlights the cultivable diversity and bioactivities of heterotrophic bacteria associated with marine macroalgae of southeast Indian coastal region. Culture-dependent isolation method resulted in 40 isolates, in which greater part of the isolates represented Gammaproteobacteria (62%) followed by that comprised of the phylum Firmicutes. One of the most active strains isolated from a macroalga, Laurencia papillosa, was characterized based on the integrated phenotypic and genotypic analysis as Bacillus velezensis MBTDLP1 MTCC 13048, with an inhibition zone of about 35 mm against methicillin-resistant Staphylococcus aureus (MRSA), was selected for bioprospecting studies. Type-I pks gene (MT394492) of 700 bp could be amplified from the heterotrophic B. velezensis. The bacterium exhibited siderophore production and possessed genes implicated in the biosynthesis of siderophore type of metabolites exhibiting 99% similarity with other GenBank sequences in BLAST search. B. velezensis exhibited promising anti-infective properties against methicillin-resistant Staphylococcus aureus (minimum inhibitory concentration 15 µg/mL), and the activities were positively correlated (r2 > 0.9) with iron-chelating activities. Chemical investigation of the organic extract of B. velezensis MBTDLP1 characterized a macrocyclic polyketide exhibiting prospective antibacterial potential against methicillin-resistant S. aureus (MIC 0.38 µg/mL), than that exhibited by positive control chloramphenicol (6.25 µg/mL). Significant antibacterial activity against drug-resistant bacteria combined with the presence of genes coding for bioactive secondary metabolites revealed that this marine symbiotic bacterium could be used against emerging antibiotic resistance.