Cleaning protocol In both hospital buildings, the same external company was hired for environmental cleaning of hospital surfaces. Both in the old and new hospital building, rooms were cleaned daily with microfiber cloths dampened with water, unless disinfection was indicated. Sinks were part of this daily cleaning routine and the protocol for sink cleaning remained unchanged during the study period. To ensure quality, internal and external audits were performed regularly. After a patient in the old building was discharged, the nightstand and bed were removed to be cleaned, but no additional cleaning measures were taken besides daily cleaning. In the new hospital building, the whole room was cleaned before a new patient could be admitted to the room. Additionally, cleaning staff received extra training after relocating. Also, in the new building, facility care workers (FCW) were introduced. Several cleaning tasks were transferred from the cleaning staff to the FCW. In general, when no disinfection was indicated, the cleaning staff was responsible for the cleaning of the built in furniture, where the FCW was responsible for the cleaning of the other equipment and furniture in the room. Statistical analyses The different patient rooms were categorized in 1) general patient rooms (i.e. two- and fourperson rooms in the old hospital building, and single-occupancy rooms on general wards in the new hospital building), 2) ICU rooms, 3) rooms with an anteroom (i.e. isolation rooms and hematology rooms), and 4) bathrooms. CFU counts per RODAC were converted into CFU counts per square cm (cm2), by dividing the CFU counts by the surface of the RODAC. CFU counts per cm2 were presented as medians. Differences between the sample moments in the old hospital building and between the two hospital buildings were analyzed using the Mann-Whitney-U test, differences within the new hospital building were analyzed with the Wilcoxon Signed-Ranks Test. Presence of HRMO was defined as yes/no, and presented with numbers and percentages, and analyzed with chi-squared analyses. P<0.05 was considered statistically significant. IBM Statistical Package for the Social Sciences Solutions (SPSS) version 25 (IBM Corp., Armonk, New York, USA) was used for all analyses. Results Colony forming units over time In total, 4993 sample sites were sampled, 724 in the old building and 4269 in the new building. RODACs were taken from 4211 out of 4993 (84.3%), 673 out of 724 (93.0%) sample sites in the old hospital building, and 3536 out of 4269 (82.8%) in the new hospital building. For nine (0.2%) sample sites the RODAC went missing in the laboratory, and the other 773 (15.5%) sample sites were bottom of sink plugs, where no RODACs were taken according to our sampling protocol. The highest median number of CFUs per cm2 was identified from the shower drain (3.95 CFUs per cm2), and the lowest from the wall (0.04 CFUs per cm2). The observed CFU counts per cm2 at both sampling moments in the old hospital building are presented in Supplementary file 3. The CFU counts determined one month before relocating to the new hospital building were used as the reference for the old hospital building (Table 1). Before relocating patients to the new hospital building, we observed 154 Chapter 3.2
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