Impacts of nanomaterials on social behaviour of microorganisms

Increasing use of nanomaterials (NMs) in a wide range of consumer products has raised concerns about their impacts on natural and engineered ecosystems. NMs released into the ecosystems will potentially influence organisms at all levels of the food chain. Microorganisms are at a low trophic level and play critical roles in maintaining the health of ecosystems. Hence, the impacts of NMs on microorganisms have attracted great interests. However, majority of the toxicological studies of NMs have focused on their bactericidal (killing) and bacteriostatic (growth-inhibiting) effects. Although the concentration of NMs in the environments is expected to be much lower than those used in most nanotoxicity studies, little is known about the influences of NMs at sublethal concentrations on critical functions of microorganisms. The objective of this study was to elucidate the impact of NMs on microbial functions. Specifically, the focus was on the influence of NMs at sublethal levels on bacterial social behaviours including production of communal resources, intercellular communication, and collective antibiotic tolerance. Cells of Pseudomonas species often produce siderophores, a group of organic compounds that facilitate iron uptake for the producers and their neighboring non-producers in microbial communities. Pyoverdine (PVD) is one of the most important siderophores produced by Pseudomonas aeruginosa and it is an important communal resource in environmental microbial communities and also a key virulence factor in establishing infections in medical settings. Tellurium nanorods (TeNRs) produced using a metal-reducing bacterium Shewanella oneidensis and physicochemically synthesized single-wall carbon nanotubes (SWCNTs) significantly inhibited PVD production in P. aeruginosa. Quantitative polymerase chain reaction (qPCR) and RNA-sequencing based transcriptomics analyses revealed that the inhibition was at transcriptional level. Quorum sensing (QS) is one of the most important signalling systems that bacteria use to coordinate cooperative behaviours. QS signalling has been shown to play a critical role in various natural and engineered bioprocesses. The acylated homoserine lactones (AHLs) is one of the predominant QS signals used by most Gram-negative bacteria. Using Pseudomonas syringae and Pantoea stewartii as model organisms, the key AHLs produced by these two organisms were identified and quantified the influences of silver nanoparticles (AgNPs) and single-wall carbon nanotubes (SWCNTs) on the production of different AHLs. P. syringae mainly produced 6 short-chain AHLs, while P. stewartii produced 11 short- and long-chain AHLs. The presence of AgNPs in P. syringae cultures resulted in overall reduction of AHLs. Intriguingly, the production of most AHLs in P. stewartii was not significantly affected by either 0.5 µg/ml AgNPs or 20 µg/ml SWCNTs. This study reports the differential effects exhibited by NMs on the production of different signals. The impact of AgNPs on collective antibiotic tolerance of a sludge wastewater community in terms of live/active community composition was also evaluated. There was no significant change in the community structure in response to the nanomaterial treatment. Moreover, treatment with tetracycline followed by AgNPs treatment did not result in significant change in the antibiotic tolerance profile under the testing conditions. Taken together, these findings provide a novel insight into environmental impacts of NMs. This study will aid in comprehensive understanding the effect of NMs on various aspects of cell-cell interactions in microbial communities that often play critical roles in maintaining the health of ecosystems in various natural and engineered ecosystems.Doctor of Philosophy (CEE