Fitness cost and biofilm formation in fosfomycin-resistant clinical Escherichia coli and Klebsiella pneumoniae isolates

Background/aim: Fosfomycin has regained importance owing to its unique mechanism of action and effectiveness against extended-spectrum β-lactamase-producing Gram-negative bacteria. This study aimed to evaluate the biological fitness cost associated with fosfomycin resistance and its impact on biofilm formation in clinical Enterobacteriaceae isolates. Materials and methods: A total of 78 Escherichia coli and 34 Klebsiella pneumoniae strains isolated from urine samples at Ege University Hospital were analyzed. Fosfomycin minimum inhibitory concentrations (MICs) were determined using the reference agar dilution method. Resistance was induced by exposing two K. pneumoniae strains with a fosfomycin MIC of 4 μg/mL and two E. coli strains susceptible to fosfomycin (MIC ≤ 8 μg/mL) to gradually increasing concentrations of the antibiotic. Biofilm-forming capacities, growth rates, and expression levels of selected virulence genes (fimH and papC in E. coli; entB, mrkD, uge, wabG, and ycfM in K. pneumoniae) were compared between variants with low and high fosfomycin MICs. Results: Of the 78 E. coli isolates, 13 (16.6%) were resistant to fosfomycin. Additionally, eight (23.5%) of 34 K. pneumoniae isolates exhibited high fosfomycin MICs (MIC > 32 μg/mL). No significant differences in biofilm formation were observed between the variants. However, the expression of the fimH gene decreased in one E. coli resistant variant compared with its susceptible counterpart. While the expression of the uge gene decreased in one K. pneumoniae isolate with a high MIC, the expression of the wabG gene increased. Slower growth rates were observed in two fosfomycin-resistant E. coli strains and one K. pneumoniae strain with a high fosfomycin MIC than in their counterparts. Conclusion: These findings suggest that, in the examined isolates, decreased susceptibility to fosfomycin was associated with slower growth, whereas biofilm formation ability remained largely unaffected. Continued surveillance of fosfomycin resistance is essential owing to its potential implications for bacterial fitness and pathogenicity.