Biocidal Mechanisms of Metallic Copper Surfaces - Use of Biocidal Surfaces for Reduction of Healthcare Acquired Infections - page 106

proteins and enzymes with low redox potential, whereas life under the new atmo-
sphere required proteins and enzymes with higher redox potentials. The later ones
were ideal for life adaptation to the new oxidizing atmosphere, therefore becoming
useful for living organisms. A new era had started, the copper era [ 15 , 32 , 68 ].
Organisms are able to take advantage of copper due to one major property:
alternate oxidation states by one electron transfer, between Cu(I) and Cu(II). This
allows handling a variety of oxidation-reduction processes [ 43 ]. Copper can func-
tion as a cofactor for a variety of enzymes involved in processes of respiration
(cytochrome c oxidase), photosynthesis (plastocyanin), reactive oxygen species
(ROS) turnover (copper-zinc superoxide dismutase), nitrite and nitrous oxide
reductases and oxygen transport (hemocyanin) [ 56 ]. In oxidases, hydroxylases,
and reductases copper acts as an electron donor/acceptor [ 56 ]. Additionally,
it can also act as an electron carrier for instance in azurin and plastocyanin [ 56 ].
In order to control the intracellular copper pool within optimal levels, organisms
developed systems regulating copper homeostasis, which will be briefly described
in the next section.
6.2.3 Copper Homeostasis
Copper is an important micro-element essential for life under an oxygenated
atmosphere, but, when in excess, becomes very toxic to cells. In order to control
intracellular copper concentration, cells developed systems that can either remove
copper from cell, sequester excess of copper or, additionally, may oxidize Cu(I) to
the less toxic Cu(II). Among Gram-negative bacteria, Escherichia coli is the best
studied microorganism regarding genes and mechanisms responsible for copper
homeostasis and resistance. E. coli possess multiple systems that confer resistance
against rising concentrations of copper ions. Surprisingly, it is not yet clear how
copper enters bacterial cells (Fig. 6.1 ). There are multiple possible ways in theory:
by diffusion across membranes, through porines across the outer membrane, or by
an unknown specific or unspecific transporter across the cytoplasmic membrane.
When copper is able to pass through the cytoplasmic membrane into cytoplasm
most of it is getting reduced to Cu(I), toxic for bacteria cells. Binding of the excess
of Cu(I) ions and activation of the expression of copper-detoxifying genes occurs
by means of CueR and CusRS, where CueR is a cytoplasmic MerR-family activa-
tor/repressor that activates expression of cueO and copA genes upon binding to
Cu(I) [ 83 ], CusRS is a periplasmic two-component system inducing the expression
of the cusCFBA operon [ 62 , 71 ], and CopA is a P-type ATPase (Fig. 6.1 ), which
transports cytoplasmic Cu(I) into the periplasm via ATP hydrolysis [ 75 ]. Removal
of the Cu(I) ions also occurs through its oxidation to Cu(II) by CueO - multicopper
oxidase [ 34 ], another mechanism of protection of the periplasmic space from
Cu (I) toxicity (Fig. 6.1 )[ 79 ]. The removal of periplasmic copper from the bacterial
cell is known to proceed through the Cus efflux system consisting of four proteins
(CusCFBA) and energized by the proton motive force (PMF) (Fig. 6.1 ). A copper