Studying the effect of nanomaterials on living cells is important since they are increasingly being used in consumer goods, say the researchers who led this new study, Peter Vikesland and Amy Pruden of Virginia Tech in the US. Sequencing the DNA of microorganisms after they have been exposed to nanomaterials is one way of doing this and it could even help in the design of nanoparticles that would be safer down the road.

Microbial communities play an important role in the ecosystem. They are now also routinely used in wastewater treatment, for example, to remove pollutants such as pathogens, pharmaceuticals and other molecules, and are thus increasingly being exposed to nanoparticles. Until now, most microbial nanotoxicity studies mainly focused on pure cultures, but assessing real-world microbial communities should be better in terms of the ecological information that can be obtained.

Metagenomics is the high-throughput sequencing of microbial community DNA extracts and, because it directly sequences this DNA, it can be used to characterize changes in the genes themselves and their function.

Gold nanoparticles ideal for this study

Vikesland and Pruden and colleagues have now employed this technique to evaluate how nanoparticles, and especially their surface coating and morphology, affect bacterial populations. The researchers looked at how microbial communities (from waste-water-activated sludge) behaved when exposed to gold nanoparticles with various surface coatings and shapes. The nanoparticles studied were either spherical or rod-shaped and were functionalized with either cetyltrimethylammonium bromide (CTAB) or polyacrylic acid (PAA).

“Gold nanoparticles were ideal for this study because they are generally inert at their core,” explains Vikesland. “This allows us to manipulate and separate out the effect of morphology (for example, of rod versus sphere shapes) and surface coating (PAA as opposed to CTAB). “In our experiments, we dosed particles with varying properties into a complex microbial community that is involved in purifying wastewater through the breakdown of ammonia and organic matter.

“Our metagenomic sequence analyses revealed that the taxonomic and functional microbial community structure underwent a unique succession pattern in the presence of gold nanoparticles, and in particular spheres, relative to controls that were not exposed to nanoparticles,” he tells

Metagenomics is a “sensitive tool”

The result illustrates that metagenomics is a very sensitive tool and that it might be used to screen novel nanomaterials in the future for their potential environmental impact and so also help in the development of more focused testing, adds Pruden. “Surprisingly, we also learnt that sphere-shaped nanoparticles impact the microbial community more than rod-shaped ones and that the shape of nanoparticles is more important than their surface coating.”

The team, reporting its work in Nature Nanotechnology doi:10.1038/s41565-017-0029-3, says that it is now busy further developing tools for metagenomics-based risk assessment – in particular with respect to antibiotic-resistance genes and their relation to environmental stressors. “For example, we want to establish benchmarks for which types of genes and which profiles in specific microbial communities may be of concern and assign relative ranking criteria to inform future testing and monitoring.”

For more research into the life cycle of nanomaterials visit the Nanotechnology focus collection.