Characteristics and Key Genetic Pathway Analysis of Cr(VI)-Resistant Bacillus subtilis Isolated from Contaminated Soil in Response to Cr(VI)

With increasing industrialization, hexavalent chromium (Cr(VI)) is used in various metal smelting and other industries, which, in turn, causes hexavalent chromium pollution. This study aimed to investigate the characteristics of isolated Bacillus subtilis (B. subtilis) from high-Cr(VI) soils and to evaluate its safety. Genomic and transcriptomic analyses were performed to explore its Cr(VI) response mechanisms, and a mouse model (24 mice) was established to evaluate the safety of the bacterium at different concentrations. Key genetic findings showed that Cr(VI) exposure significantly up-regulated the Spx gene and down-regulated the CtsR gene—two critical transcriptional regulators involved in stress response and development that mediate Cr(VI) tolerance. Pathway analysis revealed that ribosome RNA, redox balance, protein biosynthesis, metabolism, and cysteine biosynthesis play a significant role in bacterial Cr(VI) resistance. In the in vivo experiment, it was observed that the small intestine (SI), liver, and spleen of the mice remained normal without any injuries. Different levels of the F3 isolate demonstrated the ability to resist colonization by digestive juices, as observed in the SI slides. Consequently, B. subtilis can endure high levels of Cr(VI) by regulating redox process genes, which makes it a potential candidate for further research in selecting safe, tolerant, and bio-remedial isolates for Cr(VI) treatment.