Introduction Inanimate surfaces in hospitals, especially in patient rooms and bathrooms, can be a reservoir for pathogenic and possibly highly resistant microorganisms (HRMO) (1). From these environmental reservoirs, microorganisms can be transmitted to patients. Depending on the species, microorganisms are able to survive in the environment for long periods of time, ranging from a few hours up to several months or even years (2, 3). Environmental contamination of patient rooms can therefore be a prolonged source of pathogens. A review of 1,561 published outbreaks has identified that the hospital environment was the source in almost one fifth of the studied outbreaks (4). Furthermore, various studies have shown that when the previous room occupant was colonized or infected with an HRMO, subsequent patients had an increased risk for acquisition of that microorganism (5, 6). This illustrates that transmission via the environment also occurs in non-outbreak settings. Additionally, Chen et al. showed transmission from the environment to patients and vice versa for methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant enterococci, and Clostridioides difficile (7). These findings all highlight the importance of achieving a microbiologically safe hospital environment for patients. Cleaning is a key component for this, but hospital design, disinfection practices, and surface composition should be taken into account as well. New hospital designs nowadays frequently consist of either mainly or only single-occupancy rooms. Research indicates that single-occupancy rooms are an important infection prevention and control (IPC) measure, and are part of aiming for a healing environment in general (8, 9). Transitioning from multiple-occupancy rooms to single-occupancy rooms eliminates the risk of acquiring a microorganism from infected or colonized roommates (i.e. via direct or indirect contact), but not from prior room occupants (i.e. indirect contact). Currently, literature about the effect of single-occupancy rooms on environmental contamination is lacking. On May 18, 2018, the Erasmus MC University Medical Center, Rotterdam, the Netherlands (Erasmus MC), relocated from an old hospital building with mainly multiple-occupancy rooms and shared bathrooms to a newly constructed hospital building with 100% singleoccupancy rooms with private bathrooms. This provided a unique opportunity to study differences in environmental contamination between multiple- and single-occupancy rooms. We aimed to determine differences in environmental contamination between multiple-occupancy rooms and single-occupancy rooms in a non-outbreak setting, by determining the overall number of colony forming units (CFU) and the presence of HRMO on different locations in patient rooms and bathrooms. Second, we aimed to determine changes in environmental contamination of the newly constructed hospital over a threeyear follow up-period. Third, we aimed to determine if there was persistent contamination of surfaces over time by using whole genome sequencing (WGS), and to identify clusters. 3 149 Environmental contamination with MDRO in single-occupancy rooms
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