Manufactured nanoparticles can be toxic to living organisms. This work aims to
study the interaction of nanoparticles with bacteria as a model organism. The first
objective was, to determine the mechanistic pathways of nanotoxicity with an
emphasis on ions and oxidative stress as two key contributors and the second
objective, was to investigate what mechanisms bacteria have developed as a strategy
to protect themselves against nanotoxicity. The thesis further explores the role of
environmental variables such as water chemistry, organic matter and other
microorganisms, all of which can potentially change speciation of nanoparticles
through their transformation into less toxic species.
KEIO deletion mutants lacking genes encoding proteins which mediate resistance to
oxidative stress and ionic toxicity were screened and found to be sensitive to both
ionic silver and silver nanoparticles. A bioreporter to detect silver ions was
constructed. This was found not to be induced by silver nanoparticles, yet showed
reduced viability; this observation also indicates that besides ionic silver there are
other toxicity pathways. E. coli strains capable of mediating resistance to oxidative
stress by overexpression of certain proteins and bio reporters that could detect
oxidative stress were constructed. The biosensor cells provide some but not too
significant signals. Overexpression of proteins like superoxide dismutase and
catalase reduces cell growth, hence, cell viability assays do not provide a realistic
measure of protective impact, and thus this strategy is not suited to detect the nature
of nanotoxicity.
The protective role of extracellular polymeric substances (EPS) was studied by
developing an engineered strain of E. coli that overproduces the EPS colanic acid,
and use of mutant strains of Sinorhizobium meliloti, a free-living N2 fixing
bacterium. Nanoparticle exposure studies reveal that overproduction of EPS
mitigates silver nanotoxicity. EPS encapsulates the cells and leads to aggregation of
nanoparticles, as shown by microscopy and dynamic light scattering. Furthermore,
addition of xanthan, an EPS analogue also produces a similar effect. Lastly, x-ray
absorption spectroscopy (XAS) of microcosms amended with silver and zinc oxide
nanoparticles show rapid transformation of nanoparticles into corresponding oxides
and sulphides. The microcosms show a significant presence of dissimilatory sulphate
reducing bacteria (DSRB), and display only marginal change in bacterial community
composition on exposure to nanoparticles. These findings suggest that nanomaterials
will undergo changes in speciation dependent on the sediment chemistry and the
metabolic activities of bacteria in the environment. This process will influence the
impact of nanoparticles and the outcomes could be quite different from controlled in
vitro exposure studies
