patients during environmental sampling, and were most likely the source of the environmental contamination. Two (12.5%) patients were admitted to a positive room and discharged before environmental sampling was performed, and were the likely source of the contamination. One (6.3%) patient was admitted to a positive room 61 days after environmental sampling. Nine patients were admitted to the ward in another room than the contaminated room, indicating transmission on the ward possibly by staff or equipment. Discussion In this extensive study of S. aureus from patients and the environment in a large tertiary care hospital in the Netherlands, we found that isolates were highly diverse and almost exclusively consisted of MSSA. We described the dynamics of S. aureus during hospitalization and our results indicate an exogenous source for the isolate in 15.9% of patients after comparing nasal screening isolates to clinical isolates. Environmental contamination was rare and temporarily, and when found most likely caused by the patient admitted to the room at the time of sampling. Our results show that transmission most frequently occurred from the patient to the environment. We showed that almost 1/6th of patients with a positive nasal sample and a clinical sample taken during hospitalization had a possible exogenous source of S. aureus. Fifteen patients acquired S. aureus isolates in their nose, also indicating an exogenous source. While it is possible that some patients had a false-negative admission culture or were colonized at another body site (approximately 6% of MSSA carriers (16)), it is unlikely that this was the case for all patients. Patients could have carried multiple S. aureus types simultaneously. Both Wertheim et al. and Cespedes et al. have shown that ~6.6% to 10% of S. aureus carriers can carry multiple S. aureus types simultaneously (6, 15). Multiple S. aureus types could have been missed, although we analyzed all morphologically different MSSA isolates. Our results showed that the patient is the most likely source for environmental contamination and not vice versa. We did not detect any long-term presence of S. aureus in the old or new hospital building. The low contamination rate indicates that our cleaning protocols are effective in removing MSSA from contaminated surfaces. Our cleaning protocol consists of daily cleaning with dampened microfiber cloths, without added cleaning- or disinfection solution, unless disinfection is indicated (e.g. after discharge of a MRSA carrier). In the new hospital building, a final cleaning after discharge of the patient was introduced. However, given the lack of long-term presence of S. aureus in the old building, our cleaning protocol was most likely already effective for MSSA before introducing the final cleaning step. Another possibility is that S. aureus strains normally does not survive in the environment. Our sampling method could also have impacted the recovery rate of S. aureus. Sampling with cottons swabs has several advantages, such as the ability to sample all different types of surfaces (17). Nonetheless, recovery rates for S. aureus are low, and due to the difficulty in standardization of sampling, recovery rates in 2 107 Dynamics of S. aureus in patients and the hospital environment
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