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Background. Serratia marcescens, a Gram-negative organism, is a well-recognized nosocomial pathogen, especially in Neonatal Intensive Care Units (NICUs). Even if multiple point sources have been identified, the source of an outbreak often remains unknown. Because an outbreak of S. marcescens can spread rapidly, closing the Unit sometimes is necessary. Here, we report on an outbreak of S. marcescens occurred in our NICU and describe the control measures taken to stop the epidemic without closing the Unit. Material and Methods. Our Unit is a 56-bed Unit composed of two areas: a 23-bed (4 rooms) intensive-care and a 33-bed (6 rooms) intermediate-care area. After some cases of S. marcescens infection were identified during a 3-month period, a prospective epidemiological study was performed in both areas during a period of 8 months. Surveillance cultures were obtained from all neonates (pharynx, rectum, eyes, ears) at admission, at room-changing and twice weekly, from medical and nursing staff (pharynx, rectum) and from the environment (sinks, ventilators, incubators, soap dispensers, disinfectants, breast pumps, work surfaces). The following control measures were also taken: universal precautions were intensified (handwashing, gloves, masks), education of the staff was stressed, a survey was instituted to check the observance of the control measures, admissions to the NICU were limited and infected/colonized babies were strictly cohorted. Because the outbreak continued despite these control measures, we separated new admissions from hospitalized babies by using two ways in the Unit: a clean way (green) and a dirty way (red) with nurses, rooms and everything different between the green and the red babies. Results. During the study period, 589 neonates underwent surveillance cultures (14.156 samples); 32/589 (5%) infants had positive swabs. Four (12.5%) of the 32 colonized infants had clinical signs of infection: sepsis-like symptoms (2 cases) and conjunctivitis (2 cases). Twenty-two (68.8%) of the 32 colonized infants became positive in the intensive-care area; the median time from admission to the first positive culture was 13 days (range 6-83). Risk factors for infection/colonization were low birthweight (green way and the red way.Conclusions. In this outbreak, no evidence of environmental or staff reservoirs was shown. Transmission was likely horizontal, from newborn to newborn, through the hands of medical and nursing staff and the infected/colonized infants were themselves the reservoir of the pathogen. Probably, the organism was introduced into our NICU from an ex-preterm patient from another hospital (index case). The use of a clean way and a dirty way, in addition to the other control measures, appeared the most effective practice to stop the epidemic without closing the Unit.