Multidrug-resistant microorganisms (MDRO) are increasing worldwide and are leading to increased healthcare costs, morbidity, and mortality (1, 2). They are a frequent source of healthcare-associated infections (HAI), hindering antimicrobial treatment (3). HAI can have an endogenous source, i.e., body sites such as the skin, or an exogenous source, e.g., the hospital environment, its surfaces, or healthcare workers. This thesis focused on endogenous sources by screening patients upon admission to the hospital, and on exogenous sources, specifically the hospital environment, of MDRO and Staphylococcus aureus. We aimed to determine the effect of transitioning to a newly constructed hospital with 100% single-occupancy rooms and private bathrooms on the microbial safety of the hospital. We consider the microbial safety of the new hospital as improved when the environmental contamination in general and/or with MDRO is lower, and/or when the acquisition and/or transmission of MDRO is lower compared to the old hospital. The studies in this thesis were divided into patient related and environmental related research. In the patient related research chapter (Chapter 2), we determined the effect on acquisition of ESBL-E (Chapter 2.1). Additionally, we aimed to determine screening methods to identify patients colonized with MDRO upon admission (Chapter 2.2 and Chapter 2.3). Furthermore, we aimed to determine the dynamics between patients and the hospital environment for Staphylococcus aureus (Chapter 2.4). In the environmental related research chapter (Chapter 3), we performed a survey to determine current sampling practices throughout Europe (Chapter 3.1). Finally, we determined the effect of the new environment, i.e., single rooms, on environmental contamination of the patient room and bathroom (Chapter 3.2). Dynamics of MDRO and S. aureus colonization during hospitalization The main goal of infection prevention and control is to prevent or to stop spread of nosocomial MDRO throughout the hospital. For example, patients who are known carriers are cared for in isolation, and patients at risk for being a MDRO carrier are screened and cared for in isolation depending on the type of MDRO at risk. Upon identification of a new carrier, contact tracing can be performed to determine if transmission occurred. While these are important practices, there are still many knowledge gaps, including colonization rates upon admission to the hospital, and the dynamics of colonization during hospitalization. In the Netherlands, patients are not routinely screened for MDRO colonization upon admission to the hospital. Consequently, the true carriage rate upon admission is not known. Additionally, this makes it likely that MDRO carriers are not identified, and as a consequence, transmissions within the hospital go unrecognized. In Chapter 2.1 and Chapter 2.2, we have determined the carriage rates of extended-spectrum beta-lactamase (ESBL)-producing Enterobacterales (ESBL-E) upon admission to the hospital; in Chapter 2.3 we determined the colonization rate for MDRO upon admission; and in Chapter 2.4 we have determined the carriage rates for S. aureus upon hospitalization. We identified a carriage rate for ESBL-E between 4.4% and 6.5%, with no significant differences between the old and the new hospital building (4, 5). While these prevalence rates are low, they are not unexpected and are comparable to prevalence rates identified in other studies in the 4 185 Summarizing discussion
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