Biocidal Mechanisms of Metallic Copper Surfaces - Use of Biocidal Surfaces for Reduction of Healthcare Acquired Infections - page 128

including proteins and cellular DNA [ 27 , 39 ]. Although major steps of this process
have been identified, the exact mechanism of this process is still under discussion.
At the same time, Keevil and co-workers [ 86 , 87 ] proposed an alternative chain
of events, suggesting that Cu(I), Cu(II) and superoxide are responsible for killing
under wet and dry exposure; and the first event that leads to cell death is DNA
damage followed by cessation of bacterial respiration and membrane depolarization,
with no observed membrane damage [ 86 ]. However, killing experiments performed
with D. radiodurans and mutation rate experiment with E. coli [ 27 ], S. haemolyticus
[ 28 ]and S. cerevisiae [ 72 ] confirmed that DNA is not the first target of copper
surface-induced toxicity. Eventually, when cells are dead, DNA becomes degraded,
as demonstrated by the comet assay [ 27 ]. Additionally, one can assume that freshly
surface-released copper and ROS would induce toxicity to the closest biomolecules
available - the lipids. Indeed, recent experimental data suggest that lipids are
damaged first [ 39 ] followed by protein oxidation [ 63 ] and copper and ROS are
indicated to be contributors for initiation of lipid oxidation processes [ 78 ]. In fact,
cells accumulate such a high quantity of copper ions that copper-induced lipid
peroxidation seems more than likely. Considering the presence of ROS, in particular
the highly reactive hydroxyl radical (HO￿), lipid peroxidation appears to be unavoid-
able and leads to further damage through autocatalytic and self-propagating mecha-
nism [ 78 ] and is then boosted by the continuous presence of high copper-levels and
further ROS production. Additionally, oxidation is rapid, and propagates into many
different reactions, which further initiates other reactions leading to deeper lipid
degradation. These findings correlate well with the observed fast killing kinetics of
cells exposed to copper surfaces. Preliminary data from fatty acid methyl esters
(FAME) analysis revealed that the most predominant fatty acids were affected by
metallic copper exposure when compared with stainless steel surfaces (Esp ´ rito Santo
unpublished observations). Further analysis is needed to determine which lipids are
mainly targeted by the toxicity and by which reactions occur during oxidation.
Damage to the membranes also can explain the loss of respiration observed by
Keevil and co-workers [ 66 , 86 - 88 , 94 ] likely via loss of the proton motive force.
Also, respiration can be inactivated by protein oxidation [ 63 ]. Alternatively, as
suggested by Warnes & Keevil [ 86 ], some cytochromes are inhibited by copper
binding through a change in their conformation. However, this alternative seems
untimely: the first damage that causes lethality, occurs on the membranes, making the
membrane permeable [ 27 , 28 , 39 ] and uncoupling the respiratory chain. Additionally,
due to the fast killing kinetics, and given the high copper accumulation and high ROS
generation [ 25 , 27 , 28 , 72 ], toxicity should not be focused just on cytochromes but on
all components of the membrane (including the complete respiratory chain).
6.5 Metallic Copper Under Healthcare Environments
The bacteriostatic effect of copper in hospital settings was reported as early as 1983 by
Dr. Phyllis J Kuhn [ 48 ]. During a microbiology training for housekeeping and main-
tenance personnel at the Hamot Medical Center in Pennsylvania, students were given