| dc.description.abstract | Abstract
Due to the current water resource policy in Taiwan that allows irrigation canals to receive the effluent from wastewater treatment systems of nearby house-holds and/or industrial manufactures, it is reported that a considerable amount of sediment and farmland have been contaminated with chalcophile heavy metals (e.g., copper, zinc, cadmium and lead) resulting from frequent incidents of illegal discharge of untreated wastewater. While the impact of a pollutant on the environment is related to its total emitted concentration, it is the bioavailability of a heavy metal under specific environmental conditions that plays the key role in determining the potential risk exerted on the ecosystem health. Hence, a better understanding of the bioavailability of a heavy metal will lead to a more accurate assessment on the ecological health risk and more efficient environmental management and remediation. This knowledge may also provide insight into the contemporary mechanism (i.e., co-selection pathways) of antibiotic resistance development in the environment. In this study, laboratory experiments were conducted to investigate the bioavailability of Cd(II) to two phylogenetically distinct prokaryotes, namely Escherichia coli K-12 (a Gram-negative bacterium) and Bacillus subtilis 168 (a Gram-positive bacterium), using growth inhibition and cell death as an indicator of Cd(II) uptake. Specifically, chemistry of the assay medium was manipulated by varying chloride and organic carbon concentrations to test the hypothesis that the uncomplexed, free Cd(II) ion (i.e. Cd2+) is the primary permeant to the cell via active transport. In addition, cell-washing was performed to quantify the intracellular quantity of Cd(II). Results of the study were indeed consistent with the hypothesis: the increase in Cd2+ concentration resulted in increasing inhibition and death of both Gram-positive and negative bacteria. Moreover, the death rate decreased significantly in starving cells. It is thus suggested that the tested bacteria may take up Cd(II) into their cell by active transport. The obtained results also indicated that if Cd(II) mainly appears in the form of free Cd ions, it may cause higher pollution and toxicity risk to the ecosystem, and hence it is necessary to improve the quality of wastewater treatment process.
Keywords: Bioavailability, extracellular speciation, non-resistant bacteria, Cd uptake, active transport | en_US |