Immobilization of fluorescent whole cell biosensors for the improved detection of heavy metal pollutants present in aquatic environment

Fluorescent whole cell bacterial biosensors are very specific in detection of toxic heavy metal pollutants present in environmental water samples. However, detection of the actual fluorescence intensity emitted by the biosensors is a technological challenge because the biosensors remain in suspended form and there can be no control over the cell movement and multiplication. Immobilization of these biosensors is essential to develop a more stable, robust and sustainable detection system that is able to measure the actual intensity. In this report, we have established Polyethylenimine (PEI), a synthetic polymer, as the promising immobilizing agent for three fluorescent whole cell Escherichia coli DH5 alpha biosensors which detect Hg(II), Pb(II) and Zn(II) respectively within a specified range. These recombinant biosensors harbor a gene for reporter protein called GFP (Green Fluorescent Protein), whose heavy metal induced expression leads to fluorescence which is directly proportional to the concentration of the heavy metals present in the water samples. For immobilization, the log phase cultures of Hg(II), Pb(II) and Zn(II) biosensors were adsorbed on the respective semi-dried PEI matrix fixed on the glass slides. Each of the three immobilized biosensors remained viable up to 24 h without being fed with any nutrients. Thus, the immobilization efficiency of PEI in terms of sustaining the viable biosensors with stable fluorescence was found to be 86.21%, 90.11% and 87.32% for Hg(II), Pb(II) and Zn(II) biosensors respectively. Whereas, without immobilization the biosensors could retain viability and fluorescence ability for only 4 hours. Hence, PEI provides a suitable environment for the stable sustenance of all the three biosensors along with enhanced target selectivity and consistent sensitivity in detection of Hg(II), Pb(II) and Zn(II) pollutants present in environmental water samples. (C) 2015 Elsevier Ltd. All rights reserved.