Improving the sensitivity of bacterial bioreporters for heavy metals

Whole-cell bacterial bioreporters represent a convenient testing method for quantifying the bioavailability of
contaminants in environmental samples. Despite the fact that several bioreporters have been constructed for measuring
heavy metals, their application to environmental samples has remained minimal. The major drawbacks of the available
bioreporters include a lack of sensitivity and specificity. Here, we report an improvement in the limit of detection of
bacterial bioreporters by interfering with the natural metal homeostasis system of the host bacterium. The limit of
detection of a Pseudomonas putida KT2440-based Zn/Cd/Pb-biosensor was improved by a factor of up to 45 by disrupting
four main efflux transporters for Zn/Cd/Pb and thereby causing the metals to accumulate in the cell. The specificity of
the bioreporter could be modified by changing the sensor element. A Zn-specific bioreporter was achieved by using the
promoter of the cadA1 gene from P. putida as a sensor element. The constructed transporter-deficient P. putida reporter
strain detected Zn2+ concentrations about 50 times lower than that possible with other available Zn-bioreporters. The
achieved detection limits were significantly below the permitted limit values for Zn and Pb in water and in soil, allowing
for reliable detection of heavy metals in the environment.