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

<i>Klebsiella pneumoniae</i> 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 <i>Serratia marcescens</i> harboring <i>bla</i>_KPC-2. Whole-genome sequencing, analysis of antibiotic resistance genes, and phenotypic susceptibility assays of serial <i>S. marcescens</i> isolates revealed that resistance arose via a 45-nucleotide in-frame duplication within <i>bla</i>_KPC-2, yielding KPC variant 44 (<i>bla</i>_KPC-44). Notably, the evolution of <i>bla</i>_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 <i>bla</i>_KPC-44 emerging outside of <i>K. pneumoniae</i> and among the few documenting a CZA-resistance-conferring KPC variant emerging in a non<i>-K</i>. <i>pneumoniae Enterobacterales</i> 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.