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With their increased use, engineered nanomaterials (ENMs) will enterthe environment where they may be altered by bacteria and affect bacterial processes. Metallic ENMs, such as CdSe quantum dots (QDs), are toxic due to the release of dissolved heavy metals, but the effects of cadmium ions versus intact QDs are mostly unknown. Here, planktonic Pseudomonas aeruginosa PG201 bacteria were cultured with similar total cadmium concentrations as either fully dissolved cadmium acetate (Cd(CH3COO)2) or ligand capped CdSe QDs, and cellular morphology, growth parameters, intracellular reactive oxygen species (ROS), along with the metal and metalloid fates were measured. QDs dissolved partially in growth media, but dissolution was less in biotic cultures compared to sterile controls. Dose-dependent growth effects were similar for low concentrations of either cadmium salts or QDs, but effects differed above a concentration threshold of 50 mg/L(total cadmium basis) where (1) the growth of QD-treated cells was more impaired, (2) the membranes of QD-grown cells were damaged, and (3) QD-grown cells contained QD-sized CdSe cytoplasmic inclusions in addition to Se0 and dissolved cadmium. For most concentrations, intracellular ROS were higher for QD-versus cadmium salts-grown bacteria. Taken together, QDs were more toxic to this opportunistic pathogen than cadmium ions, and were affected by cells through QD extracellular stabilization, intracellular enrichment and cell-associated decay. more...

Contradictory results on quantum dot cytotoxicity exist for many types of biological systems, especially microorganisms. In this study, we compare the cytotoxicity of CdTe quantum dots (QDs) to four very different environmental bacterial strains, giving quantitative models of the growth curves for exposed organisms. The mechanisms of toxicity are explored by measuring reactive oxygen species generation by the QDs alone and investigating the oxidative damage to mutant bacteria especially sensitive to ROS. Electron microscopic examination also reveals factors that may contribute to resistance to nanoparticles in some strains. more...

Few simple labeling methods exist for simultaneous fluorescence and electron microscopy of bacteria and biofilms. Here we describe the synthesis, characterization, and application of fluorescent nanoparticle quantum dot (QD) conjugates to target microbial species, including difficult to label Gram-negative strains. These QD conjugates impart contrast for both environmental scanning electron microscopy (ESEM) and fluorescence microscopy, permitting observation of living and fixed bacteria and biofilms. We apply these probes for studying biofilms extracted from perennial cold springs in the Canadian High Arctic, which is a particularly challenging system. In these biofilms, sulfur-metabolizing bacteria live in close association with unusual sulfur mineral formations. Following simple labeling protocols with the QD conjugates, we are able to image these organisms in fully-hydrated samples and visualize their relationship to the sulfur minerals using both ESEM and fluorescence microscopy. We then use scanning transmission electron microscopy to observe precipitated sulfur around individual cells and within the biofilm lattice. All combined, this information sheds light on the possible mechanisms of biofilm and mineral structure formation. These new QD conjugates and techniques are highly transferable to many other microbiological applications, especially those involving Gram-negative bacteria, and can be used for correlated fluorescence and electron microscopy. [ABSTRACT FROM AUTHOR] more...