Viktors Vibornijs¹, Martins Zubkins¹, Anna Zajakina², Edvards Strods¹, Zhanna Rudevica², Ksenija Korotkaja², Juris Purans^1
1University of Latvia, Riga, Latvia
²Latvian Biomedical Research and Study Centre, Riga, Latvia
The development and testing of antimicrobial coatings continues to be a crucial approach, considering the ongoing emergence of antibiotic-resistant bacteria and the rapid transmission of highly pathogenic viruses. Commonly, methodologies like JIS Z 2801 and EU ISO 22196 are employed to assess these coatings' effectiveness. During testing, surfaces undergo application of a microorganism suspension, typically bacteria or viruses, with concentrations up to 1×10⁶ CFU or infection units. Incubation durations range from 1 hour to several days, followed by washing and subsequent growth assessment on solid agar or liquid media, depending on the organism type. The plaque formation test determines virus titre. Throughout testing, surfaces contact not only microorganisms but also substances like NaCl, K₂HPO₄, MgCl₂, and other inorganic compounds present in the microbiological media. These substances may directly interact with the coating, potentially influencing the reliability of antimicrobial test outcomes. For instance, NaCl may react with tungsten or molybdenum oxides, while MgCl₂ in media could react with Cu coatings. Additionally, due to the involvement of live bacteria and viruses, biosafety precautions are imperative during these testing procedures.
Various new approaches for evaluating antimicrobial efficiency were performed on magnetron-sputtered thin films and multilayer coatings - ZnO, Cu, WO₃, WO₃/Cu/WO₃, and ZnO/Cu/ZnO. Previously measured antimicrobial efficiency ranged from 0.5 to 6.0 colony formation units log-reduction rates for E.coli and S.aureus bacteria and up to 7 infection units log-reduction rates for the positive-sense single-stranded enveloped RNA virus MS2. Reactive oxygen species (ROS) formation, cell viability, and other test results were compared with direct antibacterial and antiviral testing to assess the potential application of alternative methods. In some cases, the decrease in cell viability caused by coating application correlates with antimicrobial testing results. However, the rate of ROS formation does not directly indicate the antimicrobial efficiency of coatings. The results also provide valuable information about the possible mechanisms of antimicrobial action of the tested coatings.