Investigation of a Serratia Marcescens bacteremia Cluster in a Cardiothoracic Intensive Care Unit (CICU) Utilizing Whole Genome Sequencing

Abstract

BACKGROUND Serratia bacteremia is a significant cause of morbidity and mortality in the intensive care unit (ICU). In August 2018 we identified 5 cases in our CICU, prompting an investigation for patient risk factors, exposures, and environmental sources. METHODS To ascertain common exposures among the cases, patient demographic and mortality data, unit length of stay, room location, infection, and surgical histories were collected by chart review. Medication histories were obtained from pharmacy records and the potential for drug diversion was explored. Environment of care rounds were conducted in both the CICU and the Cardiac operating rooms. Environmental culturing was considered. Four case isolates along with 21 non-cluster Serratia blood isolates from the same facility were analyzed for relatedness by whole genome sequencing (WGS) analysis. RESULTS The case age range was 30-80 years with 80% male gender. Mortality rate among cases was 60%. The investigation did not reveal any common risk factors or exposures among cases either in the CICU or the operating room. Environmental cultures were not collected. No evidence of drug diversion or medication contamination was identified. WGS analysis did not show any close genotypic relationship among the patients’ isolates. CONCLUSIONS WGS was used to investigate Serratia bacteremia isolates which were clustered in time, clinical service and location. The WGS results, however, indicated that these cases did not arise from a common source nor from cross-infection. WGS provides an unprecedented level of specificity for outbreak investigations permitting infection prevention resources to be used appropriately and cost-efficiently.