Biological Peptides, Metabolites and Nanoparticles from Potassium-Solubilizing Bacteria: A Recent Horizon in Antimicrobial and Anti-biofilm Strategies.

This study reports the isolation and comprehensive characterization of two potent potassium-solubilizing bacterial (KSB) strains, KSB-1 and KSB-5, from the rhizospheric soil of Iris kashmiriana locally named as Mazarmond which is cultivated near graveyard. Iris kashmiriana has antimicrobial and anti-inflammatory activities, being also used to treat stomach disorders and respiratory ailments. Both strains exhibited efficient potassium solubilization on mica-amended Aleksandrov medium, with KSB-1 showing the highest solubilization index (3.00 ± 0.12) and a significant pH drop in broth cultures. Morphological, biochemical, and molecular identification revealed KSB-1 as Priestia megaterium and KSB-5 as Bacillus paramycoides. Peptide profiling via sodium dodecyl sulfate - polyacrylamide gel electrophoresis (SDS-PAGE) and ultra-performance liquid chromatography (UPLC) uncovered unique antimicrobial tetrapeptides rich in arginine, histidine, and proline. Two-dimensional correlation spectroscopy - Fourier transform nuclear magnetic resonance (2D COSY-FTNMR) spectroscopy further identified diverse polar and semi-polar metabolites such as amines, ether, alcohol, aldehydes, alkanes, and carboxylic acid contributing to bioactivity. KSB-5 exhibited superior plant growth promoting (PGPR) traits, including phosphate, silicate, and zinc solubilization, siderophore and IAA production, and strong antifungal activity against Fusarium oxysporum. Anti-biofilm assays showed a 57.03% inhibition by KSB-5 extracts. Field emission scanning electron microscopy (FESEM) and energy dispersive X-ray spectroscopy (EDS) analyses revealed hyphal deformation and deposition of biogenic putative nanoparticles-titanium, selenium, and aluminum-on fungal cells, absent in controls. High resolution transmission electron microscopy (HR-TEM) further confirmed intracellular damage including cytoplasmic disintegration and membrane detachment. This is the first report linking KSB strains from Iris kashmiriana with putative nanoparticle-mediated antifungal and anti-biofilm activities, emphasizing their dual role as biofertilizers and biocontrol agents. The findings highlight their potential application in sustainable agriculture and integrated disease management.