Metabolic rewiring and morphological adaptations drive bacterial strain-specific cadmium defense in the Yangtze River estuary.

Cadmium (Cd) contamination in coastal regions poses severe environmental risks, yet bacterial defense mechanisms against Cd remain poorly understood. This study unveils distinct tolerant strategies of two highly Cd-tolerant bacteria isolated from the Yangtze River estuary: Comamonas sp. Y49 and Aeromonas sp. Y23. We exposed two bacterial strains to Cd concentrations ranging from sub-lethal to near-lethal levels, based on their minimum inhibitory concentrations, to investigate their stress response mechanisms. The cellular adaptations were comprehensively analyzed through transcriptomic profiling and scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDS). Transcriptomic analyses revealed that both strains significantly stimulated carbon, nitrogen and sulfur metabolism under Cd stress for maintaining essential substance and energy resources. They both enhanced reactive oxygen scavenger and polyamine biosynthesis gene regulation, suggesting a shared strategy for mitigating oxidative stress. Strain Comamonas sp. Y49 showed a 2.97-fold increase in metal efflux gene regulation and secreted extracellular polysaccharide-like substances with SEM-EDS detecting 0.50 % Cd on cell surfaces, while Aeromonas sp. Y23 potentially reduced Cd uptake by forming long-chain cellular structures. Besides, Comamonas sp. Y49 downregulated motility genes by 2.07-fold, while Aeromonas sp. Y23 upregulated them by 1.12-fold, indicating divergent biofilm formation strategies. This study provides novel insights into bacterial Cd resistance, revealing strain-specific adaptive mechanisms that combine metabolic rewiring, morphological changes, and molecular defense strategies. Our findings provide valuable insights on bacterial adaptations to metal stress and establish a molecular foundation for developing microbial-based strategies to address metal contamination in estuarine environments.