Genomic epidemiological investigation and control of a pseudo-outbreak of nosocomial infection associated with contamination of transport medium

This report describes the genomic epidemiological investigation and control of a pseudo-outbreak of nosocomial infection caused by contaminated transport medium, providing basics for the investigation of nosocomial infection outbreak and the control of transport medium. An epidemiological investigation was conducted into a pseudo-outbreak involving clustered detection of Acinetobacter junii and Aeromonas from open wound specimens of inpatients at hospital between August and September 2025.Environmental hygiene sampling was carried out. Whole-genome sequencing (WGS) was used to investigate genetic relationships. Resistance genes, virulence genes and plasmid replicons were identified. During the investigation, Acinetobacter junii was isolated in wound specimens from 43 patients, with the 90.7% (39/43) of specimens were detected with Aeromonas , 13.9% (6/43) of specimens were detected with other bacteria. Epidemiological investigation showed that all wound swabs of patients were transported in transport medium. After the batch of transport medium was removed from use, the pseudo-outbreak terminated. 22 isolates of Acinetobacter junii were obtained in this study, comprising 16 from patient and 6 from transport medium. Whole Genome Sequencing (WGS) revealed close clustering among the 22 isolates, which differed significantly from publicly available genomes in the National Center for Biotechnology Information (NCBI) database. Multiple isolates shared the same node, with a minimum pairwise single nucleotide polymorphism (SNP) distance of 0. SNP distances between nodes ranged from 1 to 8. Except for strain A3, which was isolated from a patient specimen, the remaining 21 isolates were clonally related. No resistance genes were detected in any isolate other than A3. All isolates carried virulence-associated genes. All 22 isolates contained MOBP-type plasmids. Contaminated transport medium was confirmed as the source of the pseudo-outbreak of nosocomial infection. Epidemiological investigations of nosocomial infection outbreak or pseudo-outbreak should consider the potential for contamination of transport medium as the cause. Not applicable.