Extracellular polymeric substances from copper-tolerance Sinorhizobium meliloti immobilize Cu²⁺

The copper tolerance gene of wild-type heavy metal-tolerance Sinorhizobium meliloti CCNWSX0020 was mutated by transposon Tn5-a. The mutant was sensitive up to 1.4mM Cu(2+). Production, components, surface morphology, and functional groups of extracellular polymeric substances (EPS) of the wild-type strains were compared with sensitive mutant in immobilization of Cu(2+). EPS produced by S. meliloti CCNWSX0020 restricts uptake of Cu(2+). The cell wall EPS were categorized based on the compactness and fastness: soluble EPS (S-EPS), loosely bound EPS (LB-EPS), and tightly bound EPS (TB-EPS). LB-EPS played a more important role than S-EPS and TB-EPS in Cu(2+) immobilization. Scanning electron microscopy (SEM) analysis LB-EPS had rough surface and many honeycomb pores, making them conducive to copper entry; therefore, they may play a role as a microbial protective barrier. Fourier transform-infrared (FT-IR) analysis further confirm that proteins and carbohydrates were the main extracellular compounds which had functional groups such as carboxyl (COOH), hydroxyl (OH), and amide (NH), primarily involved in metal ion binding.