# Carbapenem-resistant Salmonella Derby harboring a plasmid carrying blaNDM-1 from a clinical case in China

## TL;DR

A rare Salmonella strain resistant to carbapenem antibiotics was found in a patient in China, likely due to a plasmid carrying resistance genes.

## Contribution

First report of carbapenem-resistant Salmonella Derby with a plasmid carrying blaNDM-1.

## Key findings

- The isolate carried a plasmid with blaNDM-1 and qnrS1 genes, which can transfer to other bacteria.
- The plasmid showed high similarity to one from Salmonella Typhimurium in Zhejiang, China.
- Most carbapenemase-producing Salmonella strains were scattered, but some showed clonal spread.

## Abstract

The increasing antimicrobial resistance in non-typhoidal Salmonella (NTS) poses a growing challenge to clinical therapy. This study reports, for the first time, a carbapenem-resistant Salmonella enterica serovar Derby isolate. Although serovar Derby accounts for a relatively small proportion of clinical NTS infections, elucidating the mechanism, origin, and dissemination potential of its carbapenem resistance is crucial for enhancing surveillance and prevention strategies against resistant NTS.

Antimicrobial susceptibility testing was performed using commercial broth microdilution panels with the Beckman Coulter WalkAway 96 PLUS system. Whole-genome sequencing (WGS) and S1-pulsed-field gel electrophoresis (PFGE) were employed to characterize the chromosomes and plasmids of isolates. Conjugation assays were conducted to evaluate plasmid mobility. Additionally, the NCBI Genome and Pathogens databases were used to identify carbapenemase-producing Salmonella strains.

A patient with aplastic anemia was admitted with abdominal pain and received successive treatments. During periods of recurrent fever, carbapenem-resistant S. Derby (CS_CRSA) and Escherichia coli (CS_CREco) were isolated from rectal swabs. WGS revealed that both strains carried a nearly identical IncFII plasmid (80,195/80,198 bp) harboring blaNDM-1 and qnrS1 genes. This plasmid contained a complete conjugation module, and could be transferred from CS_CRSA and CS_CREco to the recipient at efficiencies of (4.50 ± 1.29)×10−2 and (3.17 ± 0.74)×10−1. Comparative analysis showed its high similarity to a resistance plasmid of Salmonella enterica serovar Typhimurium isolated from Zhejiang, China. As of June 25, 2025, 35 fully assembled Salmonella enterica strains carrying carbapenemase genes were identified, predominantly S. Typhimurium and its variants. Phylogenetic analysis indicated that most carbapenemase-producing Salmonella (CPSA) strains were scattered, while clonal dissemination was observed in some serotypes.

This study reports a clinical isolate of carbapenem-resistant S. Derby, likely resulting from horizontal transfer of a blaNDM-1-carrying plasmid, which indicates that carbapenem resistance is extending to less common and low virulence serovars of Salmonella. The emergence of such strains poses a challenge to patient care, especially for immunocompromised populations suffering from invasive infections. Additionally, clonal dissemination of CPSA in certain serotypes warrants heightened vigilance and preventive measures.