Biomedical Engineering Reference
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Fig. 6.11 Mutations assessment after
E. coli
exposure to surfaces. A number of 10
8
cells were
exposed for 5 s to copper surfaces, stainless steel surfaces, or surfaces containing 0.25 % (wt/vol)
of the mutagen formaldehyde (CH
2
O) plus stainless steel, removed, concentrated, and spread on
solid medium containing 20
g/mL of the bacteriostatic compound d-cycloserine. CFU were
counted as originating from mutation events leading to resistance via inactivation of CycA, a
d-cycloserine uptake permease. Triplicates were performed. The asterisk denotes significantly
different values (P
μ
0.05) for formaldehyde-challenged cells and standard deviations indicated as
error bars
very effective DNA repair systems enabling cells to recover from highly
fragmented genomes [
14
]. If the primary target of metallic copper toxicity is
DNA fragmentation, then
D. radiodurans
should be resistant to metallic copper
exposure. Yet, stationary-growth-phase cells of
D. radiodurans
are quickly
inactivated by metallic copper surfaces (Fig.
6.12
). Even when
D. radiodurans
cells have their maximum DNA repair capacities [
84
], in exponential-growth-
phase,
D. radiodurans
is unable to survive metallic copper stress. Furthermore,
DNA degradation was measured by the comet assay technique, which permits
observing DNA degradation at the individual cell level. In this experiment DNA
degradation was not observed before cell death but only after cell death, indicating
that the primary target is not DNA but instead another biomolecule.
6.3.3.2 Membrane Permeability
As DNA was ruled out as the primary target for metallic copper toxicity, the
bacterial cell envelope was considered to be the first structure to encounter metallic
copper-induced damage. Indeed this was proven by using the Live/Dead
®
staining
technique that entails two nucleic acid stains: a green-fluorescent SYTO
®
9 and a
red-fluorescent propidium iodide stain [
27
,
28
,
72
]. On one hand, SYTO
®
9 stain
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