Abstract
A set of nine recombinant heavy metal-specific luminescent bacterial sensors belonging to Gram-negative (Escherichia and Pseudomonas) and Gram-positive (Staphylococcus and Bacillus) genera and containing various types of recombinant metal-response genetic elements was characterized for heavy metal bioavailability studies. All nine strains were induced by Hg and Cd and five strains also by Zn. As a lowest limit, the sensors were detecting 0.03 μg·L-1 of Hg, 2 μg·L-1 of Cd and 400 μg·L-1 of Zn. Limit of determination of the sensors depended mostly on metal-response element, whereas the toxicity of those metals towards the sensor bacteria was mostly dependent on the type of the host bacterium, with Gram-positive strains being more sensitive than Gram-negative ones. The set of sensors was used to evaluate bioavailability of Hg, Cd and Zn in spiked soils. The bioavailable fraction of Cd and Zn in soil suspension assay (2.6 – 5.1% and 0.32 – 0.61%, of the total Cd and Zn, respectively) was almost comparable for all the sensors, whereas the bioavailability of Hg was about 10-fold higher for Gram-negative sensor cells (30.5% of total Hg), compared to Gram-positive ones (3.2% of the total Hg). For Zn, the bioavailable fraction in soil-water suspensions and respective extracts was comparable (0.37 versus 0.33% of the total Zn). However, in the case of Cd, for all the sensors used and for Hg concerning only Gram-negative sensor strains, the bioavailable fraction in soil-water suspensions exceeded the water-extracted fraction about 14-fold, indicating that upon direct contact, an additional fraction of Cd and Hg was mobilized by those sensor bacteria. Thus, for robust bioavailability studies of heavy metals in soils any type of genetic metal-response elements could be used for the construction of the sensor strains. However, Gram-positive and Gram-negative senor strains should be used in parallel as the bioavailability of heavy metals to those bacterial groups may be different.
Highlights
Heavy metals released into the environment both from natural and anthropogenic sources accumulate in soils and sediments, creating polluted environments and posing potential risk to soil organisms
Different bacteria belonging to both Gram-negative (Pseudomonas fluorescens and Escherichia coli) and Gram-positive (Bacillus subtilis and Staphylococcus aureus) groups and having different natural habitats - from soil to human gut microflora, were used as hosts for the sensors applied in this study (Table 1)
OS8::KnzntRPzntAlux, CadC and PcadA from Cd resistance system in pI258 of the Staphylococcus aureus were used in B. subtilis BR151(pcadCPcadAlux) and S. aureus RN4220(pcadCPcadAlux) and MerR and Pmer from broad spectrum Hg resistance system of the Serratia marcescens plasmid pDU1358 were used in E. coli MC1061(pmerRBSBPmerlux) and P. fluorescens
Summary
Heavy metals released into the environment both from natural and anthropogenic sources accumulate in soils and sediments, creating polluted environments and posing potential risk to soil organisms. Most of the soil or sediment-accumulated heavy metals tend to become tightly sorbed to soil/sediment solid matrix [1]. It has been shown that in natural water bodies, the concentrations of heavy metals in sediments are three to five orders of magnitude higher than in the overlaying water [2]. Heavy metals may sorb on phosphate minerals, hydrous oxides of aluminum, iron and manganese [3] as well as to natural organic matter [4]. In order to enter the living cells i.e., to be bioavailable, the heavy metals have first to be solubilized, because the biological effects may only be caused by soluble ionic form of metals [5]. We have designated these fractions as water-extracted bioavailable (bioavailable fraction detected with sensor bacteria in soil-water extract) and total bioavailable (detected with sensor bacteria in soilwater suspension)
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