Characterization of Mercury-Resistant Rhizobacteria for Plant Growth Promotion: An In Vitro and In Silico Approach

In this study, a total 30 rhizobacterial isolates were screened out based on resistance against different concentrations of mercuric chloride (HgCl₂), growth on nitrogen-free mannitol (NFM) and production of indole-3-acetic acid (IAA). The biochemical and plant growth promoting characterization of selected isolates was performed by different biochemical tests. Out of 30, six isolates, UM-3, AZ-5, UM-7, UM-11, UM-26, and UM-28 showed resistance at 30 µg/ml HgCl₂, pronounced growth on NFM and high production of IAA as 18.6, 16.7, 16, 18.7, 14, and 16 µg/ml, respectively (P < 0.05). The 16S rDNA ribotyping and phylogenetic analysis of selected bacterial isolates were performed and characterized as Exiguobacterium sp. UM-3 (KJ736011), Bacillus thuringiensis AZ-5 (KJ675627), Bacillus subtilis UM-7 (KJ736013), Enterobacter cloacae UM-11 (KJ736014), Pseudomonas aeruginosa UM-26 (KJ736016), P. aeruginosa UM-28 (KJ736017) and Bacillus pumilus UM-16 (KJ736015) used as negative control. B. thuringiensis AZ-5 showed high resistance against 30 µg/ml of HgCl₂ due to the presence of merB gene. The structural determination of MerB protein was carried out using bioinformatics tools, i.e., Protparam, Pfam, InterProScan, STRING, Jpred4, PSIPRED, I-TASSER, COACH server and ERRAT. These tools predicted the structural based functional homology of MerB protein (organomercuric lyase) in association with MerA (mercuric reductase) in bacterial Hg-detoxification system.