What is the best screening method to detect Serratia marcescens colonization during an outbreak in a neonatal intensive care nursery?

Serratia marcescens can cause outbreaks in neonatal intensive care units (NICUs), but the optimum method of screening for carriage has not been determined in this group of patients. During an outbreak of S. marcescens infections across two geographically distinct NICUs, babies were screened for both respiratory and gastrointestinal carriage. A total of 58 babies were colonized (N=47) or infected (N=11) with S. marcescens over a nine-month period. Forty-four colonized babies had paired respiratory and gastrointestinal tract specimens collected. Of these, 39% were positive from the gastrointestinal tract and 22% from respiratory tract specimens alone, and 39% had growth from both sites. Enrichment prior to inoculation on to solid media increased the detection of respiratory tract carriers. During an outbreak of S. marcescens, both respiratory and gastrointestinal samples should be collected in order to maximize the identification of colonized infants. Broth enrichment adds an additional day to obtaining results but increases the yield of positive results from respiratory specimens.