Transcriptomic analysis of Rhodococcus sp. strain SC26 under chromium stress

Chromium contamination poses a significant threat to both human health and ecological balance (Pushkar et al., Journal of Environmental Management, 2021). Among the various remediation strategies, biological approaches show great promise for mitigating chromium contamination, by reducing it into its less toxic form Cr(III) (Roșca et al., Processes, 2023). Gaining a deeper understanding of the metabolic response of bacteria to hexavalent chromium [Cr(VI)] is crucial for elucidating resistance mechanisms and enhancing the bioremediation of Cr(VI)-contaminated wastewater (Ramli et al., Microbiological Research, 2023). Rhodococcus sp. strain SC26 demonstrated high resistance to Cr(VI) (MIC of 4 mM). During growth in the presence of 1 mM Cr(VI), SC26 cells achieved up to 98% reduction to Cr(III) within 72 hours (Fig. 1). When tested in real wastewater containing 2mM Cr(VI), the strain maintained a reduction efficiency of 82%. NADP-dependent oxidoreductase and glutathione S-transferase were found to be overexpressed by Real Time q-PCR and enzymatic determinations. Despite the promising potential of strain SC26 in Cr(VI) reduction, detailed Cr(VI) reduction and related ROS scavenging pathways remain poorly documented within the Rhodococcus genus. This gap may be attributed to challenges in gene annotation, which can obscure potential metabolic routes. Additionally, the taxonomic complexity of the Rhodococcus genus (Garrido-Sanz et al., Microorganisms, 2020) complicates the accurate characterization of these pathways due to frequent reclassification and merging of species. To address this knowledge gap, this study aims to elucidate Cr(VI) reduction pathway in Rhodococcus sp. strain SC26 through whole-genome sequencing and transcriptomic analysis. By comparing differential gene expression profiles, this research seeks to identify key genes and regulatory networks involved in chromium detoxification, providing a theoretical basis for the bioremediation of Cr(VI) in contaminated environments. Acknowledgments: This work was supported by PSR 2021 Linea 6 1H_HUB project. A.M. is awarded by a PhD fellowship by the University of Milan - Food Systems PhD Program.