Chemical toxicity detection using quantum dot encoded E. coli cells

• Quantum dot encoded E. coli cells as fluorescent switches.
• Whole-cell fluorescent biosensor for chemical toxicity assessment.
• Detection of cellular stress.

Bioconjugated quantum dots (QDs) with E. coli cells (bioconjugates) were employed as fluorescent switches that turn-off instantly against any cellular-stress caused by a toxic chemical. Paraquat (PQ), H2O2 and triton X-100 were used as models for assessing their toxicities on bioconjugates. These chemicals interacted on the cell-surfaces where QDs are harbored. The extent of toxicity imposed by chemicals on bioconjugates was successfully probed by (i) real-time fluorescence signals, (ii) visible changes upon UV-light illumination and (iii) scanning electron microscopic (SEM) analysis. Hierarchical cluster analysis using kinetic data of fluorescence and viable cell numbers showed a close relationship between structurally different compounds having similar toxic effects, such as PQ and H2O2, both induced toxicities through generating reactive oxygen species (ROS). In contrast, triton X-100 disrupted the cell–wall integrity and thus showed distinct response due to the loss of cell-bound QDs. Increasing cellular toxicity with chemicals thus followed the order PQ < H2O2 < TX100 confirming the inherent nature of model chemicals to induce cellular toxicity. Our results demonstrated a facile optical strategy that enables rapid and real-time cytotoxicity screening of potentially hazardous chemicals, such as new drugs that lead to ROS generation.