Characterization and molecular insights of a chromium-reducing bacterium Bacillus tropicus

Environmental pollution from metal toxicity is a widespread concern. Certain bacteria hold promise for bioremediation that converts toxic chromium into a less harmful form, promoting environmental cleanup. In this study, we report the isolation and detailed characterization of a highly chromium-tolerant bacterium, Bacillus tropicus CRB14. The isolate is capable of growing on 5000 mg/l Cr (VI) in LB agar plate while on 900 mg/l Cr (VI) in LB broth with an 86.57% reduction ability within 96 hours of culture. It can also tolerate high levels of As, Cd, Co, Fe, Zn, and Pb. The plant growth-promoting potential of the isolate was demonstrated by a significant activity of nitrogen fixation, phosphate solubilization, IAA, and siderophore production. Whole-genome sequencing revealed that the isolate lacks plasmids for Cr resistance, suggesting genes reside on its chromosome. The presence of the chrA gene points towards Cr (VI) transport, while the absence of ycnD suggests alternative reduction pathways. The genome harbors features like genomic islands and CRISPR-Cas systems, potentially aiding adaptation and defense. Analysis suggests a robust metabolism, potentially involved in Cr detoxification. Notably, genes for siderophore and NRP-metallophore production were identified. Whole Genome Sequencing (WGS) data also provides the basis for molecular validation of various genes. Findings from this study highlight the potential application of Bacillus tropicus CRB14 for bioremediation while plant growth promotion can be utilized as an added benefit.