Abstract
The transfer of microorganisms via contaminated small droplets presents a vital pathway to the spread of disease. These droplets can be expelled actively by coughing and sneezing, but also via a proxy, such as splashes from contaminated sinks and toilets. Antimicrobial materials provide a preventative approach to controlling the microbial populations on surfaces to combat this issue. However, currently, there are limited standardized testing methods for assessing the antimicrobial efficacy of surfaces using small droplets in a realistic manner. Furthermore, the impact of moisture is often overlooked in current standards. Thus, a method of assessing antimicrobial materials in realistic conditions using contaminated small droplets was developed to assess the efficacy of a given material under both wetted and dry conditions. Additionally, this method was translated to determine the impact of moisture on the antimicrobial efficacy of copper surfaces against bacteria (methicillin-resistant Staphylococcus aureus [MRSA] and Pseudomonas aeruginosa), yeast (Candida albicans), and viruses (bacteriophage: Φ6 and MS2). For each microorganism, stainless steel (inert) and copper (antimicrobial) surfaces were inoculated with ten 1 µL droplets of microbial inoculum and recovered at the following intervals: immediately, at the time of evaporation of the droplets on the surface (activity when droplet/moisture present), and 2 hours post-evaporation (activity when droplet/moisture absent). Reduced survival of both P. aeruginosa and Φ6 phage was observed on stainless steel, indicating they are inappropriate for this method, likely because of desiccation. Copper was demonstrated to have a reduced antimicrobial efficacy under lower relative humidity conditions in all cases compared to reports in the literature, highlighting the benefit of a method that assesses efficacy using approaches more aligned to real-world application of these materials, demonstrating a need for careful consideration regarding the implementation of antimicrobial materials based on methods that might not simulate end-use.IMPORTANCESmall droplet transfer represents a key pathway for microbial movement throughout the built environment, contributing to increased healthcare costs and mortality rates, particularly via healthcare-associated infections. Antimicrobial materials can provide a vital passive and low-maintenance method of microbial control within a wider infection control system. However, current standards often use either environmental conditions unrealistic to the end-use environment (e.g., relative humidity above 90% that can overemphasize the efficacy of some materials) and/or methodological specifications that increase operator variability (e.g., manual spreading of inoculum on the surface). This paper describes a method of assessing the antimicrobial efficacy of a surface under both moist and dry conditions via small droplet contamination. The method was applied to antimicrobial copper surfaces that can pass current standards, such as ISO 22196, due to constant moisture availability throughout the test and found a decreased antibacterial and antifungal efficacy while still providing high antiviral efficacy.