Does the land-use impact the risk of inducing antibiotic tolerance by heavy metal pollution?

The rise of antibiotic-resistant soil microbial communities is a critical global issue. Evidence suggests that heavy metals can select or co-select for tolerance to metals and antibiotics in soil bacteria, but it is unclear if this tolerance varies with land use. We tested the potential of bacterial communities to develop resistance to copper (Cu) or tetracycline (Tet) after amending soils from pristine forests, contaminated forests, and agricultural lands with 3000 mg kg^-1 Cu and 6000 mg kg^-1 tetracycline, separately. Results showed that bacterial communities of unamended contaminated forest soils had the highest initial tolerance to Cu, while unamended agricultural soils exhibited the highest initial tolerance to tetracycline. The inducibility of bacterial resistance to antibiotics after Cu amendment varied by land use. In pristine forests, Cu amendment significantly increased microbial tetracycline resistance, as indicated by bacterial community tolerance, likely due to higher biodiversity. In contaminated forests, Cu amendment did not induce tetracycline-resistance, as indicated by unchanged bacterial community tolerance, possibly because of existing metal pollution and compromised bacterial communities by metal pollution. In agricultural soils, microbial tetracycline resistance as indicated by bacterial community tolerance developed slowly, becoming evident only after 42 days. These findings reveal significant differences in environmental risks related to soil metal pollution across different land uses, highlighting the need for systematic studies on the mechanisms of bacterial resistance to antibiotics in metal-contaminated soils due to their human health implications.