In vivo evolution of resistance to contemporary β-lactam/β-lactamase inhibitor combinations during treatment of a KPC-producing Serratia marcescens infection

ABSTRACT Klebsiella pneumoniae carbapenemases (KPCs) are a family of serine β-lactamases that confer broad antibiotic resistance by hydrolyzing virtually all β-lactam (BL) agents. Contemporary β-lactamase inhibitors (BLIs) such as avibactam were developed to neutralize the activity of KPCs and other clinically important carbapenemases. Ceftazidime-avibactam (CZA), a BL/BLI combination in which the cephalosporin ceftazidime is protected from KPC-mediated hydrolysis, demonstrated improved outcomes in early clinical use. However, CZA-resistant isolates soon emerged. Herein, we describe a challenging clinical case in which high-level resistance to CZA evolved during therapy for a complicated infection caused by carbapenem-resistant Serratia marcescens harboring bla KPC-2 . Whole-genome sequencing, analysis of antibiotic resistance genes, and phenotypic susceptibility assays of serial S. marcescens isolates revealed that resistance arose via a 45-nucleotide in-frame duplication within bla KPC-2 , yielding KPC variant 44 ( bla KPC-44 ). Notably, the evolution of bla KPC-44 not only conferred resistance to CZA but also marked cross-resistance to meropenem-vaborbactam and imipenem-relebactam while remaining susceptible to cefiderocol. To our knowledge, this report represents the first description of bla KPC-44 emerging outside of K. pneumoniae and among the few documenting a CZA-resistance-conferring KPC variant emerging in a non -K . pneumoniae Enterobacterales species. Ultimately, the evolved strain persisted despite therapy throughout a fatal clinical course, underscoring the potential for CZA selective pressure to drive treatment-emergent resistance to multiple contemporary BL/BLI agents.