Response of soil bacterial communities pre-exposed to different metals and reinoculated in an unpolluted soil

Changes in tolerance levels of soil bacterial communities pre-exposed to different metal concentrations and then reestablished in an unpolluted soil were examined during a one year incubation period using the thymidine incorporation technique. The study was performed with a sterilized agricultural soil, which was reinoculated with bacteria extracted from soils previously contaminated with high doses of Zn, Cu or Cd, and from an unpolluted soil. Bacteria pre-exposed to metal addition initially exhibited a greater tolerance than those non-pre-exposed. The microbial communities responded to the absence of metals in the reinoculated soil with a rapid decrease in community tolerance, losing most of the acquired tolerance (70–90%) within the first week. After that no changes in community tolerance were detected. Thus, a long-lasting effect of the original community tolerance of the inoculum was detected even 12 months after the metal stress was removed. At this time there was still a dose-response effect left since higher tolerance levels were usually found in soils with higher tolerant inoculum. Changes in tolerance levels over time showed similar trends, irrespective of which metal the bacterial communities were initially tolerant.