Molecular and Microbiological Characteristics of Uropathogenic <i>Escherichia coli</i> Harboring CTX-M-55

The extended-spectrum β-lactamase (ESBL) CTX-M-55, a CTX-M-15 variant distinguished by an amino-acid substitution (Ala77Val), has enhanced enzymatic activity due to higher structural stability. In Korea, CTX-M-55 remains insufficiently characterized, particularly in the context of urinary tract infections (UTIs). We identified CTX-M-55 among uropathogenic <i>Escherichia coli</i> isolates and compared its microbiological characteristics with those of CTX-M-15. In total, 247 <i>E. coli</i> isolates were collected from patients with acute pyelonephritis at Hanyang University Seoul Hospital, an 860-bed tertiary-care hospital, between July 2019 and December 2021. ESBL production was confirmed using a double-disk synergy test, and minimum inhibitory concentrations (MICs) were determined. Resistance genes were detected using PCR, and CTX-M-15 sequences were analyzed. Among 38 isolates detected using PCR, eight were confirmed as CTX-M-55 using further sequence analysis. CTX-M-55 showed (<i>P</i> >0.05) a trend toward increased resistance to aztreonam, cefotaxime, ceftazidime, and cefepime, while showing decreased resistance to amoxicillin/clavulanate and piperacillin/tazobactam. CTX-M-55 had higher MIC₅₀ values than CTX-M-15 for ceftazidime (>16 vs. 8 μg/mL), cefepime (32 vs. 1 μg/mL), and piperacillin/tazobactam (0.5 vs. 0.25 μg/mL). Virulence factors and coexisting resistance genes did not significantly differ. Our findings suggest that, given its increased resistance to ceftazidime and cefepime, CTX-M-55 should be considered when treating UTIs in Korea.