Source, carriers, and management of a Serratia marcescens outbreak on a pulmonary unit

An outbreak of Serratia marcescens was seen on a pulmonary ward from September 1999 until September 2000. During this period, there were two distinct clusters of S. marcescens isolation. In the first episode, September-October 1999, S. marcescens isolates with the same resistance pattern were isolated in 10 patients. PFGE (pulsed-field gel electrophoresis) following digestion with SpeI confirmed that these isolates were identical. After an initial decline in the number of isolates, the incidence rose again in March 2000. The resistance pattern of these isolates differed from that in 1999. PFGE showed that most of the isolates in 2000 were identical and had replaced the previous strain (strain 1). In the second episode, January-August 2000, 26 patients were colonized with the subsequent strain (strain 2). Three of these patients had serious clinical problems due to S. marcescens, two had bacteraemia and one empyema. In September 2000, strain 2 was also detected in stock solutions for inhalation therapy. After discontinuation of the use of stock solutions and emphasizing hygienic measures, the outbreak resolved. The majority (68%) of the patients positive for S. marcescens suffered from COPD (chronic obstructive pulmonary disease). PFGE results suggest that several COPD patients were carriers of the same strain of S. marcescens for a prolonged time. Re-admission of these patients could have lead to re-introduction of the epidemic strains.