Biocidal Mechanisms of Metallic Copper Surfaces - Use of Biocidal Surfaces for Reduction of Healthcare Acquired Infections - page 119

65 , 66 ]. Interestingly, both Gram-positive and -negative bacteria show a similarly
high copper accumulation [ 27 , 28 ]. Differences in survival rate coupled with
equally high copper accumulation might be due to an intrinsic feature of the
Gram-positive bacteria: a thicker cell-wall peptidoglycan layer which functions
both as a buffer and diffusion barrier for copper ions. Additionally, these cells
may also have an innate ability to resist higher degrees of desiccation (in the case
of dry-exposure) aiding the survival on copper surfaces.
Spores
Sporulation is a process that some bacteria use to escape unfavorable growth
conditions and ultimately survive. Spores are able to resist very high and very
low temperatures and extreme dehydration. These structures are able, in some
cases, to endure metallic copper toxicity [ 26 , 92 ]. However not all bacterial spores
are able to germinate after metallic copper exposure [ 26 ]. Further studies are
required to understand why some spores have the ability to escape the toxic effects
of metallic copper and others have not.
Copper Detoxifying Systems and Pre-adaptation to Copper
As discussed before, cells are able to control copper concentrations by using copper
homeostasis systems. It is expected that strains with genetically deleted copper
detoxification systems are more sensitive to metallic copper stress than their parental
strain. Surprisingly, mutated strains of E. coli were only slightly more sensitive than
their wild-type parental strain (Fig. 6.9 )[ 25 ]. Similar results were obtained with
bacterial strains from E. hirae [ 61 ], P. aeruginosa [ 22 ] and yeast strains from
C. albicans and S. cerevisiae [ 72 ]. However, when pre-incubation with non-toxic
copper concentrations was applied to parental strains, cells were able to show
enhanced survival on metallic copper (Fig. 6.10 ), but this did not prevent killing
[ 25 ]. Pre-adaptation to copper permitted cells to fully activate and produce copper
detoxifying systems, which strengthen their ability to sustain metallic copper
toxicity longer [ 25 ].
Anaerobiosis
Aerobic conditions signify the presence of oxygen, which in turn is partially
responsible for ROS production by redox active metals, such as copper and iron.
Accordingly, it is predicted that when oxygen is not present (anaerobiosis) copper
stress by ROS is limited. However, under anaerobiosis the most predominant
copper ion is Cu(I), which is the most toxic of the two ions since it is a much
stronger soft metal. When cells are exposed to metallic copper surfaces under
microaerophilic conditions (very low oxygen content), their survival is not much
higher compared to survival under aerobic conditions [ 25 ].