Increasing Fosfomycin Resistance in Klebsiella pneumoniae Isolates Causing Bloodstream Infections in Hospitalized Adults: Seven Years of Surveillance

Introduction: We investigated seven-year temporal trends in fosfomycin resistance among Klebsiella pneumoniae bloodstream isolates and characterized resistant isolates phenotypically and molecularly. Methods: We included 198 non-duplicate bloodstream isolates collected from hospitalized adults during 2017–2023. Fosfomycin MICs were determined by agar dilution and evaluated using the 2023 EUCAST intravenous clinical breakpoint, FDA susceptibility test interpretive criteria (STIC), and current EUCAST (T)ECOFF. Temporal trends were assessed by logistic regression. Phenotypic evidence of FosA-like enzyme production was assessed by PPF disk testing, acquired fosA variants by PCR and Sanger sequencing, and two representative isolates by Oxford Nanopore WGS. Results: Using the 2023 EUCAST breakpoint, 25.3% of isolates were resistant, with a significant annual increase in resistance odds (OR 1.22, 95% CI 1.04–1.44; p = 0.019). Using FDA STIC, 12.6% were resistant, with a significant temporal increase (OR 1.43, 95% CI 1.13–1.81; p = 0.003). According to the EUCAST (T)ECOFF, 6.1% were non-WT, with a non-significant trend (OR 1.36, 95% CI 0.99–1.86; p = 0.057). Among 47 viable resistant isolates, 36.2% were PPF-positive. No acquired fosA, fosA2, fosA3, or fosA4 genes were detected. Published fosA4 primers showed non-specific amplification. WGS identified chromosomal fosA5 in both sequenced isolates. Conclusions: Fosfomycin resistance increased significantly according to the 2023 EUCAST breakpoint and FDA STIC, whereas the ECOFF-based non-WT trend was not significant. The absence of common acquired fosA variants and non-specificity of published fosA4 primers highlight the need for careful molecular validation.