Tigecycline-resistant Klebsiella pneumoniae strains from sewage in Norway carry heavy-metal resistance genes encoding conjugative plasmids.

Objectives: Tigecycline is a last-resort antibiotic used for treatment of infections with carbapenem-resistant Klebsiella pneumoniae. The aim of the study was to understand the genetic mechanism of resistance and the genetic context of resistance genes in two tigecycline-resistant K. pneumoniae strains isolated from sewage in Bergen, Norway.

Methods: Complete genome sequencing of the two strains was accomplished using a combination of short-read Illumina MiSeq-based and long-read Oxford Nanopore MinION-based sequencing. Conjugation experiments were performed using filter mating and a green fluorescent protein (GFP)-tagged Escherichia coli strain.

Results: The complete genome sequences of strain K6-320.1 and strain K7-325 were assembled into two contigs for each strain, one contig representing the complete circular chromosomes of 5 223 440 bp (K6-320.1) and 5 263 092 bp (K7-325), respectively, and the other representing plasmids with sizes of 276 509 bp (pK6-320.1) and 246 731 bp (pK7-325). Strain K6-320.1 belonged to sequence type (ST)869, whereas strain K7-325 belonged to the pathogenic ST307. Both plasmids belonged to the IncFIB(K)/IncFII(K) group and carried several antibiotic resistance genes (ARGs), including tet(A) and bla. Both plasmids (pK6-320.1 and pK7-325) were transferred to a GFP-tagged E. coli strain, leading to the acquisition of resistance against multiple classes of antibiotics. Several heavy-metal resistance genes (HMRGs) encoding resistance against silver (sil), copper (pco), and arsenic (ars) were also present on both plasmids.

Conclusions: Our study demonstrates the presence of multidrug-resistant K. pneumoniae strains carrying conjugative plasmids encoding both ARGs and HMRGs that have potential for persistence in the environment and human microbiota.