OBJECTIVE To explore the cause of evolution of multilocus sequence typing (MLST) of bacteria under antibiotic conditions in the host.

METHODS Five strains of carbapenem-resistant Klebsiella pneumoniae (CRKP) were collected from a patient during treatment at the First Affiliated Hospital of Nanchang University from Jan. 2022 to Mar. 2022. Identification was performed with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and antibacterial susceptibility was tested through the VITEK-2 Compact system. Resistance and virulence genes, as well as MLST typing, were determined through whole-genome sequencing. Virulence was assessed with string tests, serum resistance experiments and a Galleria mellonella infection model. Fitness costs were analyzed through growth curves and paired competition experiments.

RESULTS After three months of anti-infective treatment, the strain in the patient transformed from the ST15 wzi19-KL19 clone to the ST11 wzi64-KL64 clone. All ST11 CRKP strains were resistant to amikacin, polymyxin and ceftazidime/avibactam. The 16S rRNA methylase gene rmtB, ISKpn74 insertion in mgrB and acquisition of the metallo-carbapenemase NDM-1 were identified as the causes of their resistance. Additionally, ST15 CRKP strains carried the kfuB virulence gene, exhibiting stronger resistance to serum killing and higher virulence. Growth curves and paired competition experiments revealed that ST11 CRKP strains had lower fitness costs under higher concentrations of meropenem, which may be a significant reason for the replacement of ST15 CRKP by ST11 CRKP during antibacterial drug therapy.

CONCLUSIONS The study describes the transformation of CRKP sequence types from ST15 to ST11 with lower fitness costs within individuals under the selective pressure of carbapenem antibacterial drugs. These changes suggest that a patient′s antibacterial drug treatment history may influence the evolution of CRKP, and effective measures should be taken to prevent its spread in clinical settings.