Antimicrobial resistance, particularly in Gram-negative bacteria, has become one of the most critical global health challenges of the 21st century. The rapid spread of carbapenem-resistant Klebsiella pneumoniae (CRKP) has greatly limited therapeutic options and highlighted the urgent need for new antimicrobial agents [1]. Tigecycline, a glycylcycline antibiotic, serves as a broad-spectrum agent effective against various multidrug-resistant pathogens, including CRKP [2]. However, the emergence of tigecycline resistance has further worsened this crisis, making this last-line treatment increasingly ineffective [1,2].

Tigecycline resistance is primarily mediated by efflux pumps and enzymatic modification. In 2010, a variant of the efflux gene tet(A) in Salmonella was found to result in low-level tigecycline resistance [3]. Yao et al. [4] identified a tet(A) variant in carbapenem-resistant hypervirulent K. pneumoniae, which possesses both multidrug resistance and high virulence characteristics, and this variant was found to mediate tigecycline resistance in carbapenem-resistant hypervirulent K. pneumoniae. Notably, in clinical CRKP isolates, a mechanism involving the amplification of a tet(A) variant has been identified, which confers a high-level tigecycline resistance phenotype [5]. In 2019, two plasmid-mediated high-level tigecycline resistance genes, tet(X3) and tet(X4), were characterized [6,7]. In 2020, a new plasmid-located efflux pump gene cluster was first reported. This gene cluster, named tmexCD1-toprJ1, enables K. pneumoniae to develop resistance to a variety of antimicrobial agents, including tigecycline [8].

The mechanisms underlying the spread of antibiotic resistance genes are complex and multifaceted. Insertion sequences (ISs) appear to be crucial in the spread of antimicrobial resistance genes across both Gram-positive and Gram-negative bacteria. When two identical ISs are positioned in the same orientation and flank DNA sequences, they can undergo recombination to form translocatable units (TUs). These TUs consist of one IS copy and the DNA segment between the two ISs, which often contains resistance genes [9,10]. Subsequently, these TUs can integrate into plasmids or chromosomal sites, thereby promoting the spread of resistance genes [9]. Less frequently, it has been reported that circular structures can also be formed through recombination of homologous sequences that lack recombinase genes [11]. These structures were provisionally named unconventional circularizable structures in 2013 [12]. The presence of these structures expands the pathways for the dissemination of resistance genes.

In this study, a K. pneumoniae strain harbouring a tet(A) variant, the tmexCD2-toprJ2 operon, and the carbapenemase gene blaNDM-1 was investigated to explore the dissemination of its associated resistance genes through the formation and integration of a circular structure.