Antibacterial activity of solid surfaces is critically dependent on relative humidity, inoculum volume and organic soiling

Antimicrobial surfaces can potentially prevent pathogen transfer between people in critical applications, e.g., health care or food industry. Antimicrobial efficacy of solid surfaces is usually assessed using wet exposure following ISO 22196 standard known to largely overestimate antimicrobial activity compared to dry conditions. While several dry test formats can be found in the literature, the semi-dry exposure scenarios e.g., oral spray or water droplets exposed to ambient environment, are less studied. Here we conducted a systematic series of experiments to determine the extent to which environmental exposure conditions influence antibacterial activity against the Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus . Five solid surfaces based on copper, silver, and quaternary ammonium compounds with known or claimed antimicrobial properties were tested in conditions mimicking microdroplet spray or larger water droplets exposed to variable relative air humidity in the presence and absence of organic soiling. All the studied environmental parameters critically affected the antibacterial activity of the tested surfaces from no effect in high organic content and low humidity conditions to higher effect in low organic content and high humidity conditions but not reaching the effect size demonstrated in ISO 22169 format. Copper proved to be the most efficient antibacterial surface across the tested conditions followed by silver and quaternary ammonium based coating. Antimicrobial testing of surfaces using spray and droplet contamination in application-relevant environmental conditions could therefore be considered as one of the worst-case exposure scenarios relevant to dry use surfaces. ### Competing Interest Statement The authors (AI and MR) are participating in the ISO Technical Committee ISO/TC 330 involved in the development of ISO/DIS 7581 method. The current study has no affiliation to ISO activities and represents authors independent approach to the issue.