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

Abstract

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 AMPconjugated 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.

Assessing the Risks of Manufactured Nanomaterials

“…Suspensions of nC 60 have been reported to exhibit antibacterial activity, although the possible mechanisms responsible for such toxicity remain unknown (17,19,21,22,36). Unlike some eukaryotic cells that can assimilate large nanoparticles (up to 100 nm) (37), bacteria generally cannot assimilate particles >5 nm, including nC 60 (38). Thus, antibacterial activity likely involves direct contact of nanoparticles with the cellular surface; this suggests that the surface chemistry and morphology of nanomaterials could be very influential factors in their toxicity.…”

Amine- and Carboxyl- Quantum Dots Affect Membrane Integrity of Bacterium Cupriavidus metallidurans CH34

“…However, there is a paucity of data concerning QDs' environmental behavior and interactions with environmentally relevant microorganisms. The limited data available are focused mainly on the toxicity of QDs and indicate that their toxicity to bacteria is to a large extent associated with the release of Cd and selenium ions from the weathering of QD cores (17), or intracellular QD concentrations (18,19). Association of QDs to alga Pseudokirchneriella subcapitata and the food chain transfer to Cerodaphnia dubia have been also recently reported (20).…”

Fluorescence Quenching of Quantum Dots by DNA Nucleotides and Amino Acids

“…[11][12][13] Influence of various compounds on the fluorescence emission properties of QD raised a strong interest in their use as fluorescent sensors, like the quenching by metal ions, [12,14,15] gold nanoparticles, [16,17] and various other compounds. [18][19][20][21][22][23] Further studies focussed on the oxidation of QD, [24] or photo-induced charge transfer, [11] and also on the influence of experimental conditions, such as temperature or pH. [7,25,26] Herein we present a systematic study on the interaction of nucleotides and amino acids with QD, which occasionally leads to a significant quenching of fluorescence intensity.…”