Quantum dots conjugated E. coli living cells as fluorescent reporters to detect cytotoxicity of chemicals

Quantum dots (QDs) have attracted a lot of interest for imaging, diagnostics and therapy due to their bright, stable fluorescence. Surface activated QDs can be conjugated to a variety of bio-active molecules for binding to bacteria or mammalian cells for various applications. In the current work we report on; (a) controlled bioconjugation of carboxylic-CdTe QDs with gram negative E. coli and (b) determining the effect of fluorescent properties of QD-E.colil bioconjugates upon exposure to toxic chemicals. For this, first the cells were bioconjugated with non-toxic, capped and water-soluble CdSe quantum dots (QD). Model toxic chemicals, such as pesticide (paraquat) and oxidative stress inducing chemical (H2O2) at concentrations ranging 0.5-5 mM were incubated with QD-E.coli bioconjugates. When these chemicals interacted with cells, the behavior of fluorescent QD-E.coli bioconjugates was altered by the stress imposed by chemicals and thus the fluorescent ability of QD-E.coli bioconjugate diminished with time and concentration of toxic chemicals. This stress is attributed to the damages occurred as a result of interaction of toxic chemicals to cell-wall or membrane of QD-E.coli bioconjugate and therefore, florescence signal was lost (Figs. 1 and 2.) The loss in fluorescence (signal off phenomena) of QD-E.coli bioconjugates can be used as probes to develop a variety of fluorescence based detection kits for the rapid determination of toxic drugs or food sample testing.