Physiological and molecular responses of chromium-resistant bacteria to Cr(VI) stress

Chromium is a redox-active metal that can exert toxic effects on living systems, particularly in its hexavalent form [Cr(VI)]. This study investigated the physiological and molecular responses of chromium-resistant bacteria isolated from electroplating effluent under Cr(VI) stress and compared them with the positive control Pseudomonas aeruginosa MTCC 7812. Morphological alterations were examined using scanning electron microscopy coupled with energy-dispersive X-ray (SEM-EDX), while functional group interactions were analysed using Fourier-transform infrared (FT-IR) spectroscopy. Gene expression analysis was performed using quantitative real-time PCR (qRT-PCR). Exposure to Cr(VI) resulted in noticeable morphological changes, including cell enlargement and surface distortion, suggesting stress-induced structural modifications. FT-IR analysis indicated shifts in characteristic absorption bands, implicating carboxyl, amino and phosphate groups in chromium interaction. The isolates were tentatively assigned to Alcaligenes faecalis (UN1) and Pseudomonas aeruginosa (LH1) based on 16S rRNA gene sequencing and high sequence similarity. qRT-PCR analysis demonstrated significant up-regulation of chromate-associated genes, with ChrA expression increasing by 10.73-fold and 1.39-fold, and ChrR by 16.91-fold and 8.90-fold in Alcaligenes faecalis UN1 and Pseudomonas aeruginosa LH1, respectively, under Cr(VI) exposure. In contrast, ChrA was down-regulated in the reference strain (0.17-fold). Antioxidant genes also showed increased expression, with superoxide dismutase (Sod) up-regulated by 2.05-fold and 1.27-fold, and glutathione-S-transferase (Gst) by 1.61-fold and 3.17-fold in Alcaligenes faecalis UN1 and Pseudomonas aeruginosa LH1, respectively. These findings provide insight into the adaptive responses of chromium-resistant bacteria to Cr(VI) stress and their potential role in chromium transformation and removal.