Antibacterial aryl-crowned polyketide from Bacillus subtilis associated with seaweed Anthophycus longifolius

Microbiological, biotechnological and chemical characterization of a previously undescribed aryl-crowned polyketide from Bacillus subtilis MTCC 10403 isolated from brown seaweed Anthophycus longifolius with activity against opportunistic Gram-negative food-borne pathogenic bacterial strains.

A culture-dependent method was used to isolate heterotrophic B. subtilis associated with A. longifolius and assessed for its antimicrobial properties. Minimum inhibitory concentration (MIC) of the title compound against the test pathogens was analysed by microtube dilution coupled with the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide-based colorimetric endpoint detection. Bacillus subtilis MTCC 10403 was found to be antagonistic against Gram-negative food-borne pathogenic Pseudomonas aeruginosa, Escherichia coli, Klebsiella pneumoniae, Salmonella enterica serotype Typhi, Aeromonas hydrophila and Vibrio sp. (diameter of zone of growth inhibition 13-22 mm). Bacillus subtilis was assessed for the presence of secondary metabolite coding polyketide synthase (pks) gene (KC589397, 700-bp gene product) and carboxylate siderophore framework in the aryl-crowned polyketide designated as 7-O-6'-(2″-acetylphenyl)-5'-hydroxyhexanoate-macrolactin by exhaustive spectroscopic techniques. The MIC assay showed that the reference antibiotics tetracycline and ampicillin were active at 25 μg ml^-1 against the test pathogens, whereas the newly isolated polyketide displayed anti-infective properties against E. coli, A. hydrophilla, P. aeruginosa and Vibrio sp. at a lower concentration (MIC <13 μg ml^-1 ). The MIC of the aryl macrolactin against K. pneumoniae was comparable with that of the referral antibiotics (~25 μg ml^-1 ). The mode of antimicrobial action of acryl-crowned macrolactin was found to be iron chelating similar to siderophores. Putative biosynthetic pathway of the pks gene product further validated its molecular attributions.

This study recognized new variant of antimicrobial aryl-crowned polyketide bearing methyl 6'-(2″-acetylphenyl)-5'-hydroxyhexanoate moiety at the C-7 position of the macrolactin system from A. longifolius-associated bacterium B. subtilis.

This study revealed seaweed-associated micro-organisms as promising biological strata to produce new-generation anti-infective agents.