Analysis of the molecular characteristics of a blaKPC-2-harbouring untypeable plasmid in Serratia marcescens

Introduction: Serratia marcescens has attracted increasing attention worldwide as a neglected opportunistic pathogen of public health concern, especially due to its antimicrobial resistance features, which usually cause nosocomial infections in immunocompromised or critically ill patients.

Methods: In our study, four carbapenem-resistant Serratia marcescens (CRSM) clinical isolates were characterized in our hospital from February 2018 to May 2018. The conjugation experiment confirmed the transferability of the carbapenem resistance gene. The types of carbapenem resistance genes were detected by PCR. The homology of the strains was analysed by pulsed field gel electrophoresis (PFGE). The characteristics of the plasmid and environment of carbapenem resistance genes were analysed after whole genome sequencing was performed. Then, we compared the amino acid sequence of the replication initiation protein and constructed a dendrogram by the neighbour-joining method.

Results: All four isolates showed carbapenem resistance conferred by a blaKPC-2-harbouring plasmid. They had exactly the same bands confirmed by PFGE and were defined as the homologous strains. The blaKPC-2 genes in all of the isolates were located in a 42,742 bp plasmid, which was located in the core region of antibiotic resistance and was composed of Tn3 family transposons, recombinant enzyme genes, ISKpn6 and ISKpn27. The core region of antibiotic resistance formed a 'Tn3-ISKpn6-blaKPC-ISKpn27-Tn3' structure, which was an independent region as a movable element belonging to transposon Tn6296 and its derivatives. The plasmid had a similar skeleton to incX6 plasmids and a similar amino acid sequence to the replication initiation protein. The plasmid was defined as an untypeable blaKPC-2-harbouring plasmid named the 'IncX6-like' plasmid.

Conclusion: The four CRSM isolates were mainly clonally disseminated with a blaKPC-2-harbouring plasmid in our hospital. The pKPC-2-HENAN1602 plasmid (CP047392) in our study was first reported in Serratia marcescens, which belongs to an untypeable group named the 'IncX6-like' plasmid. The carbapenem-resistant gene structure surrounding blaKPC-2 as a sole accessory module can be acquired by horizontal gene transfer and might lead to serious nosocomial infection.