Ecotoxicology of Manufactured Nanoparticles - Environmental and Human Health Impacts of Nanotechnology - page 292

Further evidence for the bactericidal effects of ZnO nanoparticles was obtained
by Brayner et al. (2006). These authors synthesised ZnO in di(ethylene glycol)
(DEG) medium by forced hydrolysis of zinc acetate (Feldmann, 2003) with a diam-
eter of 10.8 2.2 nm in that medium. The growth of E. coli was inhibited by 85%

on Luria-Bertani (LB) medium agar plates containing 125 mg/l ZnO nanoparticles,
although no particle characterisation was performed for the particles suspended in
LB medium. TEM studies of E. coli cells exposed to 80 mg/l ZnO nanoparticles
indicated sub-lethal changes in the cell morphology, cell membrane damage and
changes to the appearance of the cytoplasm. The latter was attributed to the inter-
nalisation of the nanoparticles but no chemical characterisation was undertaken to
confi rm the composition of the electron dense areas of cytoplasm. Reddy et al.
(2007) showed that 13 nm ZnO particles synthesised and characterised in DEG
medium (Feldmann, 2003) completely inhibited the growth of E. coli at 275 mg/l
and S. aureus at 80 mg/l on LB plates. Flow cytometry assays were used in parallel
to demonstrate a substantial loss in cell viability and membrane integrity in
nanoparticle exposed cells.
Makhluf et al. (2005) investigated the inhibitory activity of MgO nanoparticles
ranging from 8

±

2 nm to both E. coli and S. aureus in LB broth. These
particles were aggregated in the test medium with DLS measurements revealing
an agglomerate size of 350 nm, but antibacterial activity was still size dependent
with the smaller 8 nm particles being most toxic. A mechanism was proposed involv-
ing the break up of soft agglomerates at the cell surface and subsequent internalisa-
tion of individual nanoparticles (Makhluf et al. , 2005 ). E. coli was more sensitive to
MgO nanoparticles than S. aureus but both organisms were more inhibited in physi-
ological saline than in LB. In saline, even the largest 23 nm particles caused

± 1 nm to 23 ±

99%
reduction in viability within 2 h indicating that protein in the growth medium allevi-
ated the antibacterial activity of the nanoparticle. These authors suggest that the
toxic mechanism is via the production of H 2 O 2 either intracellularly (Sawai et al. ,
1996, 1998) or at the cell surface causing membrane damage, or both. Evidence for
both mechanisms is presented; the former by elevated intracellular magnesium
concentrations measured by X-ray microanalysis and the latter via a biomimetic
membrane assay showing interaction of the nanoparticles with the lipid bilayer.
Thill et al. (2006) showed toxicity of nominally 7 nm CeO 2 nanoparticles to E. coli
with 5 mg/l CeO 2 causing a 50% reduction in cell viability in 3 h. Three types of
interaction with the bacterial cells were observed, adsorption, oxidation/reduction
and toxicity. TEM images of E. coli cells exposed to CeO 2 nanoparticles (12 mg/m 2
bacterial surface) showed an outer shell of high electron density corresponding to
a layer of nanoparticles around the membrane, but this study did not determine if
the nanoparticles were internalised. It was suggested that the mechanism of toxicity
arises from intimate contact of the nanoparticles with the cell surface and biotic
reduction of Ce(IV) to Ce(III) causing oxidative stress to the cells.

>

7.5.4 Silver

The antibacterial properties of silver and its salts have been known since ancient
times. Silver was used as an aid in wound dressings until the early twentieth century