acquisition, and a significant decrease in exposure to m2. WGS showed that most patients that carried an ESBL-E at admission and discharge carried indistinguishable strains. Finally, we showed a high proportion of unknown ESBL-E carriers, of which the majority was already ESBL-E positive at admission. Only a small number of studies have determined the effect of single-occupancy rooms on HRMO acquisition, with conflicting results (10, 11, 18, 19). The only study determining the effect of single-occupancy rooms on ESBL-E acquisition was performed by Levin et al. (11). They determined that transitioning from an open plan ICU to single-occupancy rooms did not significantly decrease ESBL-E acquisition, which is similar to our results. Both Vietri et al. (18) and Ellison et al. (19), who looked at methicillin-resistant Staphylococcus aureus (MRSA) colonization and HAI with MRSA or vancomycin-resistant enterococci (VRE) respectively, found no difference after the transition to mainly single-occupancy rooms. However, our hospital transitioned to 100% single-occupancy rooms. The study of McDonald et al. (10) is the only study who also determined the effect of transitioning to a newly constructed hospital with 100% single-occupancy rooms. They determined the effect on MRSA and VRE colonization and infection, and on Clostridioides difficile infection (CDI) rates and observed that the transition did not impact CDI or MRSA infection rates, but did significantly decrease VRE colonization and infection rates and MRSA colonization rates (10). Their results indicate that transitioning to 100% single-occupancy rooms can still positively impact the acquisition of other HRMO (10). After relocating to the new hospital building, and thus after the transition to 100% singleoccupancy rooms, the number of intra-hospital patient transfers decreased significantly. The biggest decrease was seen for hematological patients. Even though hematology wards already consisted of mainly single-occupancy rooms, patients in the old hospital were often first admitted to a multiple-occupancy room, and later transferred to a single-occupancy room. Additionally, we showed an association between intra-hospital transfers and ESBL-E acquisition, with higher odds for patients who were transferred at least twice. However, there could be other explanations for these increased odds, since the need for intra-hospital patient transfers could indicate the need for additional care. Consequently, these patients might have had contact with more healthcare workers, potentially had more intravenous or arterial catheters, and a higher antibiotic consumption, which are all potential risk factors for ESBL-E acquisition. Due to the small number of patients who acquired ESBL-E, we were unable to correct for these factors. An additional benefit of the reduction of transfers could be a reduction in workload, a decrease in cost, and a decrease in medical errors (8, 20-23). As a result of the decrease in intra-hospital transfers, patients were exposed to less square meters of hospital environment in the new hospital building. Important is that not the intrahospital patient transfers in itself, but the exposure to more, and different areas of, the hospital environment is a potential source for ESBL-E. However, since the exposure to the hospital environment is related to intra-hospital transfer, the number of intra-hospital transfers during hospitalization is an important risk factor for acquisition and should be included in future studies. In 19.8% of published outbreaks, the hospital environment was identified as the source (24). Additionally, studies have shown increased odds on HRMO acquisition when the prior room occupant was infected/colonized (25). While singleoccupancy rooms in our hospital are cleaned after a patient is discharged, rooms are only 38 Chapter 2.1
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