Molecular Characterization of Carbapenem-Resistant Serratia marcescens Clinical Isolates in a Tertiary Hospital in Hangzhou, China

Abstract

Introduction: Although carbapenem-resistant Enterobacteriaceae (CRE) have been thoroughly investigated as the pathogens most commonly associated with clinical infections, data on Serratia marcescens are inadequate and superficial.

Methods: In this study, we characterized 36 carbapenem-resistant Serratia marcescens (CRSM) isolates in our hospital from April 2018 to March 2019 by analysing whole-genome sequencing (WGS) data. The molecular typing of the isolates was performed using both pulsed-field gel electrophoresis (PFGE) and core genome multilocus sequence typing (cgMLST).

Results: Thirty-three of the 36 isolates showed carbapenem resistance conferred by a bla _KPC-2-harbouring plasmid, while the remaining three isolates were characterized by overexpression of beta-lactamase combined with porin loss. The bla _KPC-2 genes in all the isolates were located on a plasmid of ~103 kb, except one, which was on a plasmid of ~94 kb. The gene structure surrounding bla _KPC-2 in the plasmids was confirmed by integration of a partial Tn4401 structure and an intact IS26 as previously reported. Most of the plasmids also contained a mobile genetic element (MGE) comprising qnr and ISKpn19, which provided evidence of horizontal transfer of antibiotic resistance genes.

Conclusion: The thirty-six CRSM isolates were mainly clonally disseminated with a bla _KPC-2-harbouring plasmid in our hospital. The gene structure surrounding bla _KPC-2 as an MGE, as well as the qnr segment, might be acquired by horizontal gene transfer, and it could aggravate the infection and increase the difficulty of clinical treatment.

Keywords: Serratia marcescens; blaKPC-2-harbouring plasmid; carbapenem resistance.

Figure 1

The Molecular typing of 36 carbapenem-resistant Serratia marcescens (CRSM) isolates by two methods. (A). Pulsed-field gel electrophoresis (PFGE) typing. The isolates were divided into 5 cluster types including cluster A, B, C, D and E based on the cutoff values. (B). The local ad hoc scheme of Core genome multilocus sequence typing (cgMLST). Isolates with no less than 14 different allelic genes were identified as follows: C60, 3717, 3460 and 3725 (in red boxes), 3024 (in green), 4201 (in purple).

Figure 2

The circular maps of pC110-KPC and p4201-KPC. (A). The circle in yellow represented plasmid pC110-KPC, in orange represented pSZF_KPC, in red represented p628-KPC. (B). The circle in purple represented p4201-KPC, in blue represented pHS091157. The peak map in (A) and (B) represented the GC content of plasmid pC110-KPC and p4201-KPC respectively. Arcs in grey indicate the position of bla_KPC and bla_qnr in plasmids pC110-KPC and p4201-KPC. The maps were created by Brig v0.95.

Figure 3

A schematic diagram of the genetic structure surrounding bla_KPC-2. The genetic structure surrounding bla_KPC-2 in the plasmid pC110-KPC represented all plasmids harbouring bla_KPC-2 in this study. The positions indicated by orange lines are the point mutation positions at which C and T are in plasmids pC110-KPC and pKPHS2, respectively. The direction of the arrow represents the direction of transcription.