Dynamic within-host cefiderocol heteroresistance caused by bla amplification in pandrug-resistant and hypervirulent Klebsiella pneumoniae sequence type 11.

Aims: Although cefiderocol (FDC) is not prescribed in China, FDC-resistant pandrug-resistant hypervirulent Klebsiella pneumoniae (PDR-hvKp) is emerging. In this study, we performed FDC susceptibility testing of clinical Kp isolates to explore the prevalence of FDC-resistant isolates and the mechanism of FDC-resistance.

Methods: We retrospectively selected 151 carbapenem-resistant Kp isolates to assess FDC susceptibility. Seven isolates harboring bla from two patients were enrolled for whole-genome sequencing. The antimicrobial resistance, virulence, bla expression, and fitness costs in different media were examined. The amplification of bla was further investigated by qPCR and long-read sequencing.

Results: The 151 isolates showed a low MIC/MIC (1/4 mg/L) of FDC. The seven isolates were ST11 PDR-hvKp, and two represented FDC-resistance (MIC=32 mg/L). The IncR/IncFII plasmids of two FDC-resistant isolates harbored 6 and 15 copies of bla, whereas four FDC-susceptible isolates carried one copy and one harbored three copies. These bla genes concatenated together and were located within the same 7.3 kb fragment flanked by IS26, which contributed to the increased expression and FDC resistance without fitness costs. The amplification of bla and FDC resistance could be induced by FDC in vitro and reversed during continuous passage.

Conclusions: The amplification of bla and the consequent dynamic within-host heteroresistance are important concerns for the rational application of antibiotics. Long-read sequencing might be a superior way to detect resistance gene amplification rapidly and accurately.