APPLIED AND ENVIRONMENTAL MICROBIOLOGY, Mar. 2011, p. 1541–1547 Vol. 77, No. 5
0099-2240/11/$12.00 doi:10.1128/AEM.02766-10
Copyright © 2011, American Society for Microbiology. All Rights Reserved.
MINIREVIEWS
Metallic Copper as an Antimicrobial Surface

Gregor Grass,1 Christopher Rensing,2 and Marc Solioz3*
School of Biological Sciences, University of Nebraska—Lincoln, Lincoln, Nebraska1; Department of Soil, Water, and Environmental Science,
University of Arizona, Tucson, Arizona2; and Dept. of Clinical Pharmacology, University of Bern, Bern, Switzerland3
Bacteria, yeasts, and viruses are rapidly killed on metallic copper surfaces, and the term “contact killing”
has been coined for this process. While the phenomenon was already known in ancient times, it is currently
receiving renewed attention. This is due to the potential use of copper as an antibacterial material in health
care settings. Contact killing was observed to take place at a rate of at least 7 to 8 logs per hour, and no live
microorganisms were generally recovered from copper surfaces after prolonged incubation. The antimicrobial
activity of copper and copper alloys is now well established, and copper has recently been registered at the U.S.
Environmental Protection Agency as the first solid antimicrobial material. In several clinical studies, copper
has been evaluated for use on touch surfaces, such as door handles, bathroom fixtures, or bed rails, in attempts
to curb nosocomial infections. In connection to these new applications of copper, it is important to understand
the mechanism of contact killing since it may bear on central issues, such as the possibility of the emergence
and spread of resistant organisms, cleaning procedures, and questions of material and object engineering.
Recent work has shed light on mechanistic aspects of contact killing. These findings will be reviewed here and
juxtaposed with the toxicity mechanisms of ionic copper. The merit of copper as a hygienic material in hospitals
and related settings will also be discussed.
The use of copper by human civilizations dates back to
between the 5th and 6th millennia B.C. It was the first metal
used, presumably because it could be found in a native, me-
tallic form which did not require smelting. Its use remained
scattered throughout Europe and the Middle East, and the
archeological evidence remains scarce. With the invention of
smelting, the metallurgic age began and the advantage of com-
bining copper with tin to form bronze was discovered. The
earliest bronze artifacts originated from the Middle East and
China and date to before 3000 B.C., but it was not until the
second millennium B.C. that bronze was used throughout
Europe. The ability to smelt and forge iron from about 1000
B.C. marks the end of the Bronze Age and the beginning of
the Iron Age.
The oldest recorded medical use of copper is mentioned in
the Smith Papyrus, one of the oldest books known (8). This
Egyptian medical text, written between 2600 and 2200 B.C.,
describes the application of copper to sterilize chest wounds
and drinking water (8). Greeks, Romans, Aztecs, and others
also used copper or copper compounds for the treatment of
such ailments as headaches, burns, intestinal worms, and ear
infections and for hygiene in general. In the 19th century, a
new awareness of copper’s medical potency was spawned by
the observation that copper workers appeared to be immune to
cholera in the 1832 and subsequent outbreaks in Paris, France
(8). The use of copper in medicine became widespread in the
19th and early 20th centuries, and a variety of inorganic copper
preparations were used to treat chronic adenitis, eczema, im-
petigo, scrofulosis, tubercular infections, lupus, syphilis, ane-
mia, chorea, and facial neuralgia (8). The use of copper as an
antimicrobial agent continued until the advent of commercially
available antibiotics in 1932. The spread of antibiotic resis-
tance through selective pressure began and today has made
antibiotic-resistant bacteria ubiquitous in hospitals, nursing
homes, food processing plants, and animal breeding facilities.
This has raised the need for different approaches to keep
pathogenic microorganisms at bay. One such alternative is the
use of copper surfaces in hygiene-sensitive areas. While this
approach is not novel (7), it had lost importance and accep-
tance in the last few decades. A 1983 report documenting the
beneficial effects of using brass and bronze on doorknobs to
prevent the spread of microbes in a hospitals remained largely
unnoticed (18). Similarly, the idea of using copper vessels to
render water drinkable has been revived only very recently as
a low-cost alternative for developing countries (37). Currently,
there is an intense interest in the use of copper as a self-
sanitizing material, and many recent publications deal with
mechanistic aspects of “contact killing” (contact-mediated kill-
ing) by copper.
COPPER AS A TOXIC BUT ESSENTIAL
TRACE ELEMENT
Copper is an essential trace element in most living organ-
isms, and more than 30 types of copper-containing proteins are
known today. Prominent examples are lysyl oxidase, which is
involved in the cross-linking of collagen, tyrosinase, required
for melanin synthesis, dopamine
-hydroxylase, which func-
tions in the catecholamine pathway, cytochrome c oxidase, the
terminal electron acceptor of the respiratory chain, and super-
* Corresponding author. Mailing address: Dept. of Clinical Pharma-
cology, University of Bern, Murtenstrasse 35, 3010 Bern, Switzerland.
Phone: 41 31 632 3268. Fax: 41 31 632 4997. E-mail: marc.solioz@ikp
.unibe.ch.

Published ahead of print on 30 December 2010.
1541

oxide dismutase, required for defense against oxidative dam-
age. In these enzymes, copper serves as an electron donor/
acceptor by alternating between the redox states Cu(I) and
Cu(II) (15). Other copper proteins, such as plastocyanins or
azurins, act as electron carriers. Depending on the type of
coordination of the copper to the protein, the redox potential
of copper can vary over the range 200 mV to 800 mV. On
the other hand, the redox properties of copper can also cause
cellular damage. A number or mechanisms have been sug-
gested. Reactive hydroxyl radicals can be generated in a Fen-
ton-type reaction:
Cu  H2O2 3 Cu2  OH  OH
(1)
The extremely reactive hydroxyl radical can participate in a
number of reactions detrimental to cellular molecules, such as
the oxidation of proteins and lipids (45). Copper ions can also
lead to depletion of sulfhydryls, such as in cysteines or gluta-
thione, in a cycle between reactions 2 and 3:
2 Cu2  2 RSH 3 2 Cu  RSSR  2H (2)
2 Cu  2 H  O2 3 2 Cu2  H2O2 (3)
The hydrogen peroxide thus generated can in turn participate
in reaction 1 and lead to further generation of toxic hydroxyl
radicals. It is still not clear to what extent reactions 1 to 3 cause
copper toxicity. Cells try to keep H2O2 at very low levels, and
reaction 1 may not be the chief toxic mechanism, although this
has been frequently claimed. An alternative route of copper
ion toxicity has been shown to be the displacement of iron from
iron-sulfur clusters (20). Similarly, copper ions may compete
with zinc or other metal ions for important binding sites on
proteins. The toxic effect of copper on microbes is utilized in
agriculture for the control of bacterial and fungal diseases (4),
which in fact led to the first thorough investigation of bacterial
resistance to copper ions (5).
Bacteria evolved a range of mechanisms to protect them-
selves from the toxic effects of copper ions: extracellular se-
questration of copper ions, relative impermeability of the outer
and inner bacterial membranes to copper ions, metallothio-
nein-like copper-scavenging proteins in the cytoplasm and
periplasm, and active extrusion of copper from the cell. The
latter appears to be the chief mechanism of copper tolerance in
bacteria and has been extensively studied in Gram-positive and
Gram-negative bacteria. In Escherichia coli, the CopA copper-
transporting ATPase resides in the cytoplasmic membrane and
pumps excess Cu(I) from the cytoplasm to the periplasm (32).
In the periplasmic space, the multicomponent copper efflux
system CusCFBA and the multicopper oxidase CueO control
the copper level and redox state, respectively. In addition to
these chromosomally encoded systems, E. coli strains can har-
bor related, plasmid-encoded systems which further increase
copper tolerance (33). All the components of this copper de-
toxification machinery are transcriptionally upregulated by
copper via two regulatory circuits. In Gram-positive bacteria,
which are devoid of a periplasmic space and an outer mem-
brane, only CopA-type copper exporters are present and a
single regulatory circuit usually controls their expression (34,
36). A number of other components, like copper-binding pro-
teins, copper reductases, etc., support these basic defense sys-
tems against copper and have been described elsewhere (2, 16,
22, 35).
In contrast to copper defense, copper utilization by bacteria
is much less well understood. In Synechocystis, it appears that
a special copper uptake ATPase serves in supplying copper to
the photosynthetic components in the thylakoid membranes
(38). On the other hand, methanotrophic bacteria that require
copper for particulate methane monooxygenase secrete sid-
erophore-like substances, the methanobactins, to scavenge ex-
tracellular copper (2). In Gram-negative bacteria, like E. coli,
it is believed that the metalation of cuproenzymes takes place
in the periplasmic space and does not require special copper
uptake systems across the cytoplasmic membrane. Finally,
many novel proteins of unknown function which are regulated
by copper have been identified in the Gram-positive organism
Lactococcus lactis (21), and further efforts will be required for
an in-depth understanding of copper handling by bacteria.
CONTACT KILLING IN THE LABORATORY
The study of the antimicrobial properties of metallic copper
surfaces is a relatively recent development and gained mo-
mentum when the Environmental Protection Agency (EPA)
registered almost 300 different copper surfaces as antimicro-
bial in 2008 (http://www.epa.gov/pesticides/factsheets/copper
-alloy-products.htm). Prior to that, a number of studies have
already dealt with the kinetics of contact killing upon exposure
of bacteria to copper and copper alloy surfaces (14, 28, 29, 43,
44). Table 1 summarizes the species tested, test procedures,
and killing kinetics. In general, microbes were inactivated on
copper within hours, but such parameters as the inoculation
technique, incubation temperature, and copper content of the
alloy used were not usually investigated in a systematic way and
are difficult to compare between studies. Nevertheless, a few
general principles appear clear: higher copper content of alloys
(43), higher temperature (10), and higher relative humidity
(25) increased the efficacy of contact killing. Treatments that
lowered corrosion rates, e.g., application of corrosion inhibi-
tors or a thick copper oxide layer, lowered the antimicrobial
effectiveness of copper surfaces (9).
In most studies on contact killing, a “wet” inoculation tech-
nique was used by applying typically 20 l of cell suspensions
to coupons. While this is a valid approach for laboratory test-
ing, it might not mimic well the dry copper surfaces encoun-
tered in health care environments. In an alternative “dry”
method, a small volume of liquid is applied to coupons with a
cotton swab. The thin film of liquid evaporates within seconds
and allows direct contact of all cells with the metal surface.
Under these conditions, E. coli and other bacteria were inac-
tivated within a few minutes of exposure (11–13). This suggests
that dry metallic copper surfaces are even more antimicrobial
than moist ones, which raises interesting questions about the
mechanism of contact killing.
Certain Gram-positive bacteria, such as members of the
Bacilli and Clostridia, form endospores which can resist heat,
radiation, desiccation, denaturing chemicals, etc. Thus, endo-
spores pose a real challenge to aseptic procedures. Clostridium
difficile is an important pathogen of the group of spore-forming
bacteria and leads to diseases like diarrhea and colitis. Excre-
tion of endospores by infected persons might contaminate sur-
1542 MINIREVIEWS APPL. ENVIRON. MICROBIOL.