Case Commentary: Emergence of clinically relevant fidaxomicin-resistant Clostridioides difficile strains

Clostridioides difficile infection (CDI) has limited antibiotic treatment options, with fidaxomicin being the guideline-preferred antibiotic. C. E. Kaple, S. N. Redmond, J. L. Cadum, B. Hausman, et al. (Antimicrob Agents Chemother 0:e00130-26, 2026, https://doi.org/10.1128/aac.00130-26) describe a case of clinical failure in an elderly patient with CDI treated with a pulsed-taper fidaxomicin regimen, in which the MIC increased from 0.05 μg/mL at baseline to 32 μg/mL in isolates recovered during relapse. Relapsing symptoms were eventually resolved only after switching to oral vancomycin. This case, together with other recent clinical reports, may be the tip of the iceberg, signaling that fidaxomicin-resistant C. difficile can arise during treatment and contribute to clinical failure. It further challenges the assumption that high colonic drug concentrations mitigate resistance and underscores the need for improved surveillance and the development of clinically applicable molecular diagnostics to detect fidaxomicin resistance in CDI patient samples.