Biocidal Mechanisms of Metallic Copper Surfaces - Use of Biocidal Surfaces for Reduction of Healthcare Acquired Infections - page 111

confirming the antimicrobial efficacy of copper surfaces versus steel control
surfaces. In addition, earlier that year, conference papers by Harold Michels and
colleagues [ 58 , 59 ] reported the killing kinetics of various copper alloys against
E. coli O157:H7, an enterohemorrhagic strain often found in ground beef. More-
over, this was the first study pointing out that temperature played a role in the
killing process: inactivation of cells on 99 % pure copper occurred within 1.5 h at
20 C, whereas lowering temperature to 4 C prolonged the killing time up to 3 h.
Non-copper containing surfaces failed to inactivate E. coli O157:H7. Another
parameter, copper concentration in different alloys, was shown to have direct
correlation with killing rate of studied surfaces: diminishing the copper content in
the alloys was accompanied by a reduction of the killing rate. These results were
published by Wilks and co-workers [ 93 ] and were in accordance with the ancient-
knowledge that early civilizations had applied but not understood, such as using
copper vats to store drinking water.
Later on many studies followed that focused on establishing killing efficiencies
of copper surfaces versus control surfaces against a variety of microorganisms
(Table 6.1 ). Typically, under wet conditions, microorganisms are killed within
hours on metallic copper surfaces (Table 6.1 ). These studies resulted in registration
of almost 300 different copper alloys as antimicrobial by the Environmental
Protection Agency (EPA) in 2008 ( http://www.epa.gov/pesticides/factsheets/cop
per-alloy-products.htm ) .
Although a substantial amount of data concerning the antimicrobial efficacy of
copper against multiple microbes was obtained prior to 2008, not much attention
was paid to the elucidation of the bacterial killing mechanism underlying such
process. Only in 2008 an initial step was made by Esp ´ rito Santo et al. [ 25 ], towards
understanding the mechanisms of metallic copper surface-mediated killing of
bacteria. For the first time, an alternative method was developed to mimic touch
to dry surfaces, where cells in a minimal buffer volume are applied directly on the
surface. Evaporation of the liquid occurs very rapidly, within seconds, mediating
immediate contact between bacterial cells and surface. This method may be applied
as a laboratory model to simulate bacteria spread through touch to surfaces, air
particles in hospitals or other public places, and air conducts. The bactericidal
properties of copper surfaces using dry copper exposure model were tested against
the panel of microorganisms (Table 6.1 ). Typically organisms are all inactivated
within minutes, highlighting much faster killing kinetics using this inoculation
technique [ 25 ].
Data obtained from studies using wet and dry methods suggest that they
employ different toxicity mechanisms resulting in unique killing kinetics as
observed by, e.g., [ 22 , 25 , 61 , 93 ]. As a result of wet inoculation, there is no
direct contact between bacterial cell and copper surface. In order to achieve the
desired antimicrobial effect, copper ions have to be released directly into the buffer
suspension [ 61 ], where their concentration has to reach a certain level to be toxic
[ 23 , 61 ]. Consequently, cells are inactivated as a result of deadly concentrations of
copper ions and copper-induced stress [ 61 ], typically within hours [ 22 , 54 , 61 , 65 ,
67 , 87 , 88 , 90 , 93 , 94 ].