Is there a widespread clone of Serratia marcescens producing outbreaks worldwide?

BACKGROUND: Serratia marcescens frequently causes outbreaks in healthcare settings. There are few studies using high-throughput sequencing (HTS) that analyse S. marcescens outbreaks. We present the analysis of two outbreaks in neonatal intensive care units (NICUs) in hospitals from the Comunitat Valenciana (CV, Spain) and the impact of using different reference genomes. METHODS: DNA from cultured isolates was extracted and sequenced by HTS using Illumina NextSeq. Reads were mapped against two reference genomes, strains UMH9 and Db11, and the unmapped fraction of the genomes was assembled to fully genetically characterize the isolates. FINDINGS: Isolates from the first outbreak were identical to the UMH9 reference, an unrelated isolate obtained three years earlier in the USA. This did not occur when the Db11 strain, a standard reference for S. marcescens, was used as the reference for mapping. To check whether UMH9 was a widely distributed clone spreading in the CV, the second outbreak isolates were mapped against this reference. They were not closely related to this strain, and this outbreak could be defined as such regardless of the reference used for mapping the reads. CONCLUSIONS: The choice of the reference for genomic analysis of outbreaks is a critical decision. In the case of the first outbreak, this choice changed the interpretation of the results drastically, allowing or preventing the definition of the outbreak according to the reference used. Although HTS is a powerful tool for epidemiological analysis, it is still essential to collect microbiological and epidemiological data for the correct interpretation of the results.