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Applied and Environmental Microbiology, May 2005, p. 2548-2557, Vol. 71, No. 5
0099-2240/05/$08.00+0     doi:10.1128/AEM.71.5.2548-2557.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Uptake of CdSe and CdSe/ZnS Quantum Dots into Bacteria via Purine-Dependent Mechanisms

J. A. Kloepfer, R. E. Mielke, and J. L. Nadeau*

Center for Life Detection, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109

Received 12 October 2004/ Accepted 3 December 2004

Quantum dots (QDs) rendered water soluble for biological applications are usually passivated by several inorganic and/or organic layers in order to increase fluorescence yield. However, these coatings greatly increase the size of the particle, making uptake by microorganisms impossible. We find that adenine- and AMP-conjugated QDs are able to label bacteria only if the particles are <5 nm in diameter. Labeling is dependent upon purine-processing mechanisms, as mutants lacking single enzymes demonstrate a qualitatively different signal than do wild-type strains. This is shown for two example species, one gram negative and one gram positive. Wild-type Bacillus subtilis incubated with QDs conjugated to adenine are strongly fluorescent; very weak signal is seen in mutant cells lacking either adenine deaminase or adenosine phosphoribosyltransferase. Conversely, QD-AMP conjugates label mutant strains more efficiently than the wild type. In Escherichia coli, QD conjugates are taken up most strongly by adenine auxotrophs and are extruded from the cells over a time course of hours. No fluorescent labeling is seen in killed bacteria or in the presence of EDTA or an excess of unlabeled adenine, AMP, or hypoxanthine. Spectroscopy and electron microscopy suggest that QDs of <5 nm can enter the cells whole, probably by means of oxidative damage to the cell membrane which is aided by light.

* Corresponding author. Present address: McGill University, Department of Biomedical Engineering, 3775 University Street, Montréal, QC H3A 2B4, Canada. Phone: (514) 398-8372. Fax: (514) 398-7461. E-mail: jay.nadeau{at}mcgill.ca.

Applied and Environmental Microbiology, May 2005, p. 2548-2557, Vol. 71, No. 5
0099-2240/05/$08.00+0     doi:10.1128/AEM.71.5.2548-2557.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.



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