Ecotoxicology of Manufactured Nanoparticles - Environmental and Human Health Impacts of Nanotechnology - page 273

could pass through bacterial cell walls and membranes: nonspecifi c diffusion, non-
specifi c membrane damage, and specifi c uptake. It should be noted that the largest
globular proteins observed to pass through intact bacterial cell walls are typically
4 nm diameter (Demchick and Koch 1996).

7.3.2 Nanoparticle Interactions with Cells: Cellular Uptake

Many classical models of toxicity assume that a toxicant must be taken up by the
cell (usually via some kind of receptor) before a toxic response may be observed
(Paquin et al. , 2002). Because of their small size, nanoparticles have generally been
considered to be more bioavailable than corresponding bulk materials (Colvin,
2003). The most likely and widespread mechanism by which nanoparticles may be
taken up by eukaryotic cells is by receptor-mediated endocytosis (Colvin, 2003;
Moore, 2006; Nowack and Bucheli, 2007), leading to their deposition in the cyto-
plasm and association with intracellular organelles. While work on the physiology
of nanoparticle uptake in relation to the study of ecotoxicology is limited, it is
instructive to consider studies involving mammalian cell lines and bacteria.
Nanoparticle uptake has been demonstrated with a number of mammalian cell lines
(Shukla et al. , 2005 ; Chithrani et al. , 2006), although this is not necessarily associated
with enhanced cytotoxicity (Shukla et al. , 2005 ).
Nanoparticles may also be internalised by mechanisms other than endocytosis,
for example passive diffusion or specifi c transport. Xu et al. (2004) used localised
surface plasmon resonance spectroscopy (LSPRS) and dark-fi eld optical micros-
copy to show silver nanoparticles up to 80 nm transporting in and out of bacterial
cells ( Pseudomonas aeruginosa ). This was surprising as there is no evidence that
prokaryotes possess mechanisms of endocytosis, and the largest pore sizes known
for specifi c transport mechanisms in bacteria are generally considered to be less
than 6 nm (Kloepfer et al. , 2005). However, membrane transport of nanoparticles
by a mechanism other than endocytosis is supported by both in vivo and in vitro
studies with mammalian cells (Geiser et al. , 2005 ; Rothen - Rutishauser et al. , 2006 ).
Geiser et al. (2005) found inhaled ultrafi ne titanium dioxide particles distributed
among different lung compartments and intracellularly localised mainly in the
cytoplasm, but particles within cells were not membrane bound indicating endocy-
tosis was not the mechanism of uptake. Further studies using red blood cells as a
model for non-phagocytic cells showed fl uorescent spheres, titanium dioxide and
gold nanoparticles

200 nm were able to penetrate into the cells by diffusion or
adhesive interaction (Rothen-Rutishauser et al. , 2006 ). Attention should therefore
be paid to the mechanism of nanoparticle uptake, as intracellular particles which
are not membrane bound may have direct access to intracellular proteins, organ-
elles and DNA and hence enhanced toxic potential (Geiser et al. , 2005 ).
A few studies have demonstrated nanoparticle uptake by aquatic organisms
(Table 7.1). In addition, a number of nanoparticles have been visualised in the
cytoplasm of bacterial cells by TEM, including silver (Figure 7.8a), (Morones et al. ,
2005 ; Xu et al. , 2004), metal oxides (Makhluf et al. , 2005 ; Brayner et al. , 2006 ) and
quantum dots (Kloepfer et al. , 2005). As yet, few data exist from ecotoxicological
studies to demonstrate intracellular nanoparticle uptake by endocytosis or other

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