Clustering of Serratia marcescens infections in a neonatal intensive care unit

OBJECTIVES: To study clusters of infections caused by Serratia marcescens in a neonatal intensive care unit (NICU) and to determine risk factors for S. marcescens infection or colonization. DESIGN: Genotyping of S. marcescens isolates was performed by pulsed-field gel electrophoresis (PFGE). A retrospective case-control study was conducted. SETTING: A tertiary-care pediatric hospital with a 16-bed NICU. PATIENTS: All neonates with at least one culture positive for S. marcescens in the NICU during December 1999 to July 2002. Case-patients (n = 11) treated in the NICU during December 1999 to February 2000 were included in the case-control study. Neonates treated in the NICU for at least 72 hours during the same period with cultures negative for S. marcescens were used as control-patients (n = 27). RESULTS: S. marcescens was cultured from 19 neonates; 9 were infected and 10 were colonized. PFGE analysis identified three epidemic strains; each cluster consisted of identical isolates, except one isolate in the first cluster that was different. The risk factors identified were low birth weight, prematurity, prolonged respiratory therapy, prolonged use of antibiotics, and maternal infection prior to delivery. Overcrowding and understaffing were recorded simultaneously with the clusters. CONCLUSIONS: PFGE analysis showed three independent clusters. Several factors contributed to spread of the epidemic strains: (1) there were many severely premature and susceptible neonates, (2) the NICU was overcrowded during the clusters, and (3) transmission was likely to occur via the hands of staff. Cohorting and improvement of routine infection control measures led to the cessation of each cluster.