Netherlands. Arcilla et al., identified a carriage rate of 6.1% in healthy volunteers and Kluytmans-van den Bergh et al, identified a carriage rate at admission to the hospital between 6.4 and 7.4% (6, 7). Additionally, in a trans-European cross-sectional study, a carriage rate of 6% was identified (8). Interestingly, the carriage rates in the Netherlands remained almost constant over the years. When compared to surveillance data for E. coli, the percentage of resistant isolates was between ranged between 6.4% and 7.5% between 2016 and 2021, with a prevalence of 6.6% in 2021 (9). Among the 1155 included patients, we did not identify patients positive for vancomycin-resistant E. faecium (VRE), multi-drug resistant Pseudomonas aeruginosa, multi-drug resistant Acinetobacter baumannii, or carbapenemase-producing Enterobacterales upon admission. The prevalence of these MDRO are low in the Netherlands, and consequently it is not unexpected that we did not identify carriers (9). One patient (0.1%) was positive for methicillin-resistant S. aureus (MRSA) upon admission to the hospital, which is in line with the prevalence of 0.07-0.13% (10, 11). Carriage rates for methicillin-susceptible S. aureus (MSSA) was 27.0%, which is in line with other studies (10, 11). During hospitalization, acquisition or loss of MDRO and/or S. aureus can occur. These dynamics are relatively unknown, as patients are not routinely screened for colonization upon admission and during hospitalization. We have determined these dynamics for ESBLE in Chapter 2.1 and for S. aureus in Chapter 2.4. We observed that 19 out of 597 (3.2%) of patients acquired an ESBL during hospitalization, of which 18 patients (94.7%) were not identified through clinical cultures. Whole genome sequencing (WGS) was performed on all available isolates, and showed no transmission between identified patients. Consequently, the source was either a patient who was not included in the MOVE-study, the hospital environment, or less probable, health care workers or visitors. Another possibility is that the admission culture was a false-negative culture due to very low abundance which was increased by antibiotic use during admission period and therefore became detectable at discharge. Other studies determining transmission of ESBL-E during hospitalization showed transmission rates of 1.5% to 5.4% (7, 12-15). In contrast to our study, the ESBL-E status as described in the published studies, were known by healthcare workers, and therefore, in most studies patients were cared for in isolation upon identification of ESBL-E. Consequently, their transmission rates cannot be directly extrapolated to unidentified carriers who are not cared for in isolation during their hospitalization, as in our study. Although our observed acquisition rate of 3.2% is comparable to the transmission rates in the other studies, we have not shown transmission between patients. It is not likely that no transmission between patients occurred. Our inability to show transmission is a consequence of the fact that we were not able to include all patients admitted to the hospital. It is reasonable to conclude that transmissions between patients were missed. In Chapter 2.4, we determined the dynamics of S. aureus during hospitalization. We showed acquisition in 15 out of 673 (2.2%) patients. Additionally, we determined if S. aureus identified in study samples were identical to clinical samples, or otherwise if they were 186 Chapter 4
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