Isolation and characterization of endosymbiotic bacteria from copper contaminated soils in Chile

Abstract

Legume endosymbiotic bacteria indigenous of copper (Cu)-contaminated soils from Chile have been isolated using pea (Pisum sativum), bean (Phaseolus vulgaris) and alfalfa (Medicago sativa) as trap host plants. Highly contaminated soils only produced nodules in certain legume hosts, whereas nodulation was observed in the three legume hosts when inoculated with soils containing a low Cu concentration. A collection of 362 strains was isolated, and their levels of Cu resistance were tested in media supplemented with increasing metal concentrations and in disk diffusion assays. By these two approaches, 84 strains displaying levels of Cu resistance higher than those exhibited by the corresponding reference strains were selected. The most resistant strains isolated from alfalfa and bean nodules grew normally at 3 mM and 2.5 mM CuSO4 and were obtained from two different highly contaminated soils. Strains nodulating pea plants showed similar levels of resistance to Cu (2-2.5 mM CuSO4) and were isolated from low-contaminated soils. Our data suggest a reduction of microbial diversity in agricultural Cu-contaminated soils from Chil