<i>Exiguobacterium mexicanum</i> PY14 Reduces Toxic Selenite to Elemental Selenium: Characterization and Mechanism

Microbial reduction of toxic Se(IV) oxyanions to biogenic Se(0) has garnered considerable attention for detoxification. This study presents a comprehensive investigation of Se(IV) reduction by environmentally versatile Exiguobacterium genus through integrated physicochemical, genomic, and transcriptomic analyses. Exiguobacterium mexicanum PY14 demonstrated remarkable efficiency, reducing ∼1 mM selenite to extracellular Se(0) within 12 h under aerobic conditions, with broad adaptability to pH (7-9), temperature (30-37°C), and salinity (up to 40 g L^-1 NaCl). The produced Se(0) revealed crystalline nanoaggregates with biomolecular coatings. Genomic sequencing identified a chromosome and six plasmids enriched with genes for carbohydrate metabolism, inorganic ion transport, and mobile genetic elements. Transcriptomic profiling under Se(IV) stress unveiled a coordinated stress response: up-regulation of catabolic pathways (glycolysis and citric acid cycle) for energy and NAD(P)H production, bacterial motility, and chemotaxis, alongside down-regulation of energy-intensive biosynthetic processes. Notably, genes for glutathione biosynthesis (gsh), NAD(P)H generation (gntZ), and ROS scavenging (btuE) were significantly up-regulated, along with the evidence of increased GSH levels, implicating a GSH-dependent detoxification pathway driving Se(IV) reduction. These findings deepen mechanistic understanding of Se(IV) reduction mechanism within the understudied Exiguobacterium genus, and the strain's haloalkaliphilic trait underscores its potential for bioremediating Se(IV)-contaminated saline-alkaline environments.