Longitudinal surveillance of carbapenem-resistant Serratia marcescens in neonatal intensive care units: transmission routes and effective control strategies.

BACKGROUND

Serratia marcescens is a known cause of nosocomial outbreaks in neonatal intensive care units (NICUs). In early 2021, an outbreak occurred in our NICUs, prompting the implementation of rigorous infection control measures. This study aimed to assess whether the outbreak strain was eliminated or persisted with sporadic transmission.

METHODS

Ongoing surveillance for S. marcescens infections was conducted in the NICUs and pediatric intensive care unit (PICU) through June 2023. Nosocomial cases were defined as infections diagnosed more than three days after hospitalization. Whole-genome sequencing, average nucleotide identity and minimum spanning tree analyses were employed to assess genetic relatedness.

RESULTS

A total of 44 infections were identified: 2 pre-outbreak, 14 during the outbreak, and 28 post-outbreak. Phylogenetic analysis of 35 isolates revealed six distinct clusters. Cluster 1 included pre-outbreak cases; Cluster 2 encompassed outbreak isolates, with one sporadic isolate detected in the PICU 18 months later. Clusters 3-6 emerged post-outbreak period and were genetically unrelated to the outbreak strain. The outbreak strain harbored multiple resistance genes, including kpnH, which potentially contributed to carbapenem resistance. Only one infection over the subsequent two years was genetically linked to the original outbreak strain, supporting the effectiveness of infection control interventions.

CONCLUSIONS

Prompt and stringent infection control measures effectively contained the S. marcescens outbreak. Continued genomic surveillance is essential to trace transmission and prevent recurrence.