Reduction of Hexavalent Chromium by Stenotrophomonas and Bacillus

Hexavalent chromium [Cr(VI)] is a widespread environmental pollutant, posing a significant health risk to ecosystems and humans. Bioremediation using microorganisms offers a cost-effective strategy for its detoxification. This review highlights recent advances in Cr(VI) reduction by Stenotrophomonas and Bacillus species, two bacterial genera with strong potential for chromium detoxification. Stenotrophomonas species primarily rely on intracellular enzymatic reduction mechanisms, often mediated by chromate reductases such as ChrR and heme proteins that link chromium detoxification with iron homeostasis. In contrast, Bacillus species employ a broader range of strategies, combining intracellular and extracellular enzymatic reduction, biosorption, and bioaccumulation, supported by stress-response and efflux systems that confer exceptional tolerance to Cr(VI). Comparative analysis reveals complementary metabolic strengths: Stenotrophomonas excels in rapid enzymatic detoxification, while Bacillus offers long-term stability through spore formation and surface-associated sequestration. Together, these traits underscore the promise of mixed consortia featuring both genera for scalable and resilient chromium bioremediation systems. Future research integrating omics-guided pathway mapping, microbial engineering, and biosafety control is expected to accelerate the deployment of efficient and safe Cr(VI) bioremediation technologies.