Abstract
Accumulation of trace elements (including heavy metals) in soil from usage of superphosphate fertilisers induces resistance of soil bacteria to trace elements of environmental concern (TEoEC) and may co-select for resistance to antibiotics (Ab). This study aimed to investigate selection of co-resistance of soil bacteria to Cd, Zn and Hg, and Ab in soils with varied management histories. Genetic diversity of these bacteria and horizontal transfer of Cd resistance genes (cadA and czcA) were also investigated. Soils with either pastoral and arable management histories and either high levels of Cd and Zn, or indigenous bush with background levels of these TEoEC from the Waikato region, New Zealand were sampled. Plate culturing with a range of TEoEC and Ab concentrations, Pollution Induced Community Tolerance (PICT) assay, antibiotic sensitivity, terminal restriction fragment length polymorphism (TRFLP) and horizontal gene transfer (HGT) analyses were employed to investigate co-selection of TEoEC and Ab resistance. Higher levels of bacterial resistance to TEoEC and Ab correlated with higher levels of TEoEC in soil. Bacterial community structures were altered in soils with high TEoEC levels. Cd resistance genes were transferred from donor bacterial isolates, to recipients and the transconjugants also had resistance to Zn and/or Hg and a range of Ab.
Highlights
Most trace elements have a fundamental role in the life processes of microorganisms and higher organisms and act as essential elements in the environment
We aimed to gain a further understanding of trace elements of environmental concern (TEoEC) resistance (TEoECR) and antibiotic resistance (AbR) co-selection by comparing ratios of TEoECR and AbR bacteria to total colony forming units in samples from sites with pastoral and arable farming land use histories with elevated soil TEoEC levels
This study showed that the abundance of the bacterial isolates resistant to TEoEC and Ab was greater in soils with higher levels of TEoEC, compared to those from soils with lower levels of TEoEC
Summary
Most trace elements have a fundamental role in the life processes of microorganisms and higher organisms and act as essential elements in the environment. Trace elements (TEs) of environmental concern (TEoEC) including cadmium (Cd), zinc (Zn) and mercury (Hg) can be added to the environment by a variety of different human-related activities or natural processes [1]. E.g., cobalt (Co), Zn and copper (Cu), are micronutrients and considered necessary elements for cellular metabolism and other processes in life. These can be harmful at high concentrations [3]. The presence of high concentrations of non-essential metals in cells can be detrimental to cell functioning, as the non-essential metals may have a higher binding affinity to thiol groups or oxidised organic products and displace essential metals [5]
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