262888 Invited: Biological Consequences of Engineered Nanoparticle Interactions with Environmental Microbial Communities
One of the many drivers for nanotechnology development is the potential for suitably designed nanomaterials to interact with and guide biological systems. By virtue of their size, their often unique physical and chemical properties and their relative ease of functionalization for specific biomolecular recognition, nanoparticles hold great promise in medicine and biotechnology. This potential for strong biological interaction, combined with considerable realized progress in biofunctional nanosystem development and significant commercial deployment of nanomaterials in a variety of markets, has prompted research efforts aimed at providing the scientific basis for nanomaterial risk assessment. While this research has not surprisingly lagged behind nanomaterial and nanotechnology research and development, the new fields of “nanotoxicology” and “nanoecotoxicology” are growing rapidly. One source for potential concern is inadvertent release of biologically potent nanoparticles that were engineered for biomedical applications. There is also a more general concern that inadvertent release into the environment of nanoparticles that were engineered for other purposes may have deleterious ecological or biological consequences. One subject of considerable contemporary research attention concerns the implications of the fairly common use of silver nanoparticles in clothing and other consumer products, because of their broad spectrum biocidal activity. Pathways have been identified by which these nanoparticles may migrate via municipal wastewater treatment plants from households to farm fields and surface waters, where they might exert as yet unknown ecological effects. Similarly, zero valent iron nanoparticles (“nanoscale zero valent iron” or NZVI) have been engineered for in situ remediation of groundwater contamination by chlorinated solvents or toxic metals. Although they are still in the research and development stage and not yet widely deployed, these nanoparticles are highly reactive and designed to be injected into groundwaters in high concentrations. As with silver nanoparticles, their actual or expected ecological effects are unknown.
Much of the research into potential environmental impacts of engineered nanoparticles concerns their effects on microbes, especially bacteria. This is because bacteria reside at the base of ecological food nets, and they play a critical role in molecular and elemental cycling in the environment. Significant disruption of microbial communities could therefore lead to significant ecosystem disruption. This presentation will focus mostly on the effects of exposing environmental bacterial communities to engineered nanoparticles, particularly NZVI and silver nanoparticles. Both NZVI and silver nanoparticle exposure have been demonstrated in laboratory culture experiments to be toxic to bacteria. The majority of these culture experiments consider planktonic, or freely suspended, bacterial cultures. In the environment, bacteria in the planktonic state are relatively rare compared to the vast majority of bacteria that reside in biofilm communities. These are adhered to solids and are protected by a secreted extracellular matrix. As of yet, there is little understanding of how these environmentally relevant community structures influence bacterial susceptibility to engineered nanoparticle exposure. Most planktonic culture experiments minimize the complexity of the bacterial surroundings. Real ecological samples have complex compositions, and their effects on nanoparticle/microbe interactions are as yet just beginning to be understood. The environment presents nanoparticles with many surfaces on which to adhere and macromolecules that may adsorb to their surfaces. These surfaces could function as sinks and adsorbed macromolecules could alter nanoparticle colloidal stability, transport and the surface forces they experience at biological cell walls.
The impact of NZVI on bacterial communities present in trichloroethylene-contaminated aquifers was studied in microcosms, where nanoparticles were added to slurries of the aquifer solids and their associated bacterial communities. These microcosm studies indicated no overall biocidal effect of NZVI, but the water chemistry changes caused by NZVI redox reactions did alter the microbial species balance. Communities shifted in favor of sulfate reducing and methanogenic organisms. Depending on the type of macromolecules used to disperse NZVI, it was found in some cases to have an overall biostimulatory effect.
The effects of the biofilm growth mode on bacterial susceptibility to silver nanoparticles were studied. Silver nanoparticles present more than one potentially toxic mode of interaction with bacteria, since dissolved silver ions are toxic. The relative degree to which toxicity is caused by direct nanoparticle action on cells or by dissolved silver ion effects can differ from one type of organism to another. Single species biofilms of Pseudomonas fluorescens were cultivated and exposed to poly(vinylpyrrolidone)-stabilized silver nanoparticles. Biofilm viability was decreased by silver nanoparticle exposure in a concentration-dependent manner, but to a far smaller degree than was observed for planktonic cultures of this bacterium. Nanoparticle concentrations that eliminated all detectable viability in planktonic culture left a significant fraction of viable bacteria in biofilms. Evidence for a nanoparticle-specific toxic effect was observed and found to correlate with the colloidal stability of the nanoparticles. Biofilm susceptibility to silver nanoparticle exposure was partially mitigated by the presence of humic acid, a representative form of natural organic matter common to most environments. This suggests a possibly important protective role for natural organic matter in the environment.
While most of this presentation will focus on nanoparticle effects on bacterial communities, the presentation will conclude with evidence that bacterial communities present in the environment are capable of acting on the nanoparticles themselves, altering them in a way that would affect their transport in the environment. In particular, evidence will be shown that bacterial communities can biodegrade polymeric stabilizers of the sort that are frequently attached to nanoparticle surfaces to control their colloidal stability. Stabilizer biodegradation was found to drive nanoparticle aggregation. It is apparent that nanoparticle/bacterial interactions “go both ways.” This will likely drive a significant amount of future research using systems engineering approaches to support environmental nanoparticle risk assessment.