Resumo

Copper mining and the application of copper-based agrochemicals have led to an increase of copper concentration in soils. Bacterial communities from three Cu-polluted soils and a non-polluted soil from Valparaíso region, central Chile, were studied by denaturing gradient gel electrophoresis (DGGE) of the 16S rRNA genes. Cu-resistant bacteria were isolated to study copA gene, resistance to other heavy metals and the presence of plasmids. The copA gene was detected in metagenomic DNA from soil by PCR. Minimum inhibitory concentration to heavy metals for bacterial strains was studied in low phosphate medium. Plasmid extraction was performed using Kado and Liu method. In Cu-polluted and non-polluted soils a similar DGGE banding patterns were observed. DGGE from the four soils showed complex profiles suggesting a low variation in the bacterial operational taxonomic units (OTUs). The copA genes encoding for the multi-copper oxidase were detected only in polluted soils. Five isolates that showed high Cu resistance (ranged from 3.1 to 4.7 mM Cu²⁺) possess the copA genes. Gram-negative isolates O12, A32 and A55 showed resistance to Co²⁺, Ni²⁺, Zn²⁺ and Hg²⁺ and belong to the genus Sphingomonas. Stenotrophomonas sp. C21 showed, in addition, resistance to CrO4^2-. The Gram-positive isolate Arthrobacter sp. strain O4 showed resistance to Co²⁺, Ni²⁺ and CrO4^2-. Plasmids carrying copA gene were detected in the four Gram-negative strains. However, the copA gene was detected only in genomic DNA of the Gram-positive strain. This study suggests that dominant bacterial groups from microbial community of agricultural soils from central Chile exposed to Cu were adapted by acquiring plasmids carrying Cu genetic determinants. Therefore, plasmids may allow the dissemination of Cu resistance genes in the bacterial community. Acknowledgements: MECESUP FSM0710 postdoctoral (LAR) and CONICYT PhD (FA, GB) fellowships and USM (131109, 130948, 130836), CN&BS and FONDECYT (1110992, 3090071) grants.