Decreased susceptibility to cefepime/zidebactam among carbapenemase-producing Escherichia coli from Stockholm, Sweden with alterations in PBP2

Objectives: We aimed to investigate the in vitro activity and genetic determinants of decreased susceptibility (DS; MIC > 4 mg/L) to cefepime/zidebactam of carbapenemase-producing Escherichia coli.

Methods: Clinical isolates (N = 150) of carbapenemase-producing E. coli (CP-EC) belonging to seven distinct STs, isolated at a university clinical microbiology laboratory during 2019-2023 in Stockholm, Sweden were included. MICs for cefepime/zidebactam were determined using the broth microdilution method and interpreted using the tentative EUCAST clinical breakpoints (Susceptible; MIC < 4 mg/L; based on cefepime breakpoint). Whole genome sequences of the isolates were analysed with an emphasis on identifying alterations in PBPs 2 and 3.

Results: Of the 150 isolates, 145 (96.6%) isolates had MICs <4 mg/L indicating susceptibility and 5 (3.3%) had MICs >4 mg/L. MICs for zidebactam alone among the five isolates with DS to cefepime/zidebactam were ≥8 mg/L. WGS analysis revealed that these five isolates were NDM-5 producers and belonged to ST405 (n = 1), ST410 (n = 2) and ST648 (n = 2). Presence of four-amino-acid inserts (YRIK/YRIN) in PBP3 was observed in 80/150 (53.3%) isolates, and mutations leading to alterations in PBP2 were observed in 41/150 (27.3%) isolates. Presence of Other β-lactamases (CTX-M group) and/or cephalosporinases (blaCMY) did not have an impact on the susceptibility to cefepime/zidebactam. Three of the five isolates with DS had a V522I substitution in PBP2.

Conclusions: Our results indicate that DS to cefepime/zidebactam among clinical isolates of E. coli could arise due to targeted mutations in PBP2.