Selective antibiofilm properties of nano-ZnO and nano-ZnO/Ag coated surfaces

Background: Spread of pathogenic microbes and antibiotic-resistant bacteria in healthcare settings and public spaces is a serious public health challenge. Materials and surface-treatments that prevent solid surface colonization and biofilm formation or impede touch-transfer of viable microbes could provide means to decrease pathogen transfer from high-touch surfaces in critical applications. Both, ZnO and Ag nanoparticles have shown a great potential in antimicrobial applications. Although antimicrobial properties of such nanoparticle suspensions are well studied, less is known about nano-enabled solid surfaces.

Results: Here we demonstrate that solid surfaces coated with nano-ZnO or nano-ZnO/Ag composites possess species-selective medium-dependent antibiofilm activity against Escherichia coli MG1655, Staphylococcus aureus ATCC25923 and Candida albicans CAI4. Colonization of nano-ZnO surfaces by E. coli and S. aureus was decreased in oligotrophic (nutrient-poor, no growth) conditions with E. coli showing higher sensitivity to Ag and S. aureus to Zn, respectively. Nano-ZnO inhibited bacterial biofilm formation in a dose-dependent manner in oligotrophic conditions reaching maximum of 2.12 and 3.49 log reduction on dense nano-ZnO surface compared to uncoated surface after 72 h for E. coli and S. aureus, respectively. Minor to no effect was observed for bacterial biofilms in growth medium (nutrient-rich, supporting exponential growth). Addition of Ag to the sparse nano-ZnO surfaces had transient negative effect on E. coli biofilm formation in oligotrophic conditions with an additional 0.5-1.6 log reduction in harvested viable cells (3-48 h post-inoculation, respectively) compared with sparse nano-ZnO without added Ag. This additional reduction decreased to a non-significant 0.34 log by 72 h. Inversely, compared to uncoated surfaces, nano-ZnO surfaces enhanced biofilm formation by C. albicans in oligotrophic conditions by 1.27 log increase in viable attached cells at 48 h time point and just a minor transient negative effect was seen in nutrient-rich medium. However, enhanced C. albicans biofilm formation on nano-ZnO surfaces in oligotrophic conditions was effectively counteracted by the addition of Ag.

Conclusion: Our results not only showed that nano-ZnO and nano-ZnO/Ag coated solid surfaces have the potential to effectively decrease surface colonization by the bacteria E. coli and S. aureus but also indicated the importance of the use of application-appropriate test conditions and exposure medium in antimicrobial surface testing. Possible selective enhancement of biofilm formation by the yeast C. albicans on Zn-enabled surfaces should be taken into account in antimicrobial surface development.

This work was funded by Estonian Research Council Grants EAG20, PRG749.