Inorganic Cadmium Detection Using a Fluorescent Whole-Cell Bacterial Bioreporter

Cadmium pollution has become a serious environmental issue due to its toxicity and frequent entrance into environment components such as soil, water, and air via many anthropogenic sources. Over the last two decades, whole-cell bacterial bioreporters have been acknowledged as bio-sentinels for the determination of toxic heavy metals. Herein a sensitive and quite specific bacterial bioreporter was developed to cope with the need for the rapid and simple determination of cadmium. The construction and characterization of a fluorescence-based whole-cell cadmium bioreporter strain, Escherichia coli MG1655 (pBR-PzntA), was described which is based on the expression of green fluorescent protein under the control of the zntA gene promoter of heavy metal resistance determinant. The developed bioreporter was able determine cadmium at 5 µg/L after 3.5 hours of induction in a defined medium while the cadmium detection limit was improved to 2 µg/L after 1.5 hours by the use of an inorganic phosphate-limiting defined medium. Drastic changes in cadmium sensitivity were obtained between bioreporter cells induced at different growth phases. The maximum fluorescence performance was obtained for early exponential growth phase cells. This cadmium bioreporter was found to be more sensitive and specific to cadmium ions than to a wide range of heavy metals and was sensitive to only cadmium at drinking water quality standard concentrations. These findings will lead to future studies including integration of the bioreporter cells into a portable device to assess bioavailable cadmium levels in environmental samples which will provide a rapid and practical field detection technique.