Evaluation of fosfomycin susceptibility using CLSI vs EUCAST criteria among multi drug resistant uropathogens in a tertiary care Hospital

Background and Objectives: Urinary Tract Infections (UTIs) are most frequently caused by uropathogenic Escherichia coli, which accounts for approximately 80% of the cases. Other causative agents include Klebsiella spp., Proteus spp., En- terobacter spp., Enterococcus spp., and Staphylococcus saprophyticus. The main objectives of the study were to estimate the in vitro antimicrobial activity of fosfomycin against multidrug-resistant uropathogens (MDR) isolated from patients with suspected UTI using CLSI and EUCAST criteria and to describe the antimicrobial susceptibility pattern of uropathogens isolated during the study. Materials and Methods: This was a descriptive study in which a total of 900 urine samples were collected from patients presenting with physician-assessed signs and symptoms suggestive of a UTI. Only samples exhibiting significant bacteriuria that were also multidrug-resistant (MDR) were included. Although fosfomycin disk diffusion criteria, according to CLSI and EUCAST, are only validated for E. coli, susceptibility among other Gram-negative bacteria was also interpreted using the same criteria. This represents a major limitation of the study. Results: In the study, 251 samples grew multi drug resistant organisms. Only 57% of the Gram-negative isolates were sensi- tive according to EUCAST guidelines, while 87.6% of all isolates were sensitive by CLSI criteria. Among the 161 carbapen- em-resistant isolates, 135 (83.9%) were fosfomycin-susceptible and 18 (11.2%) were resistant according to CLSI. In contrast, by EUCAST criteria, only 40 (24.9%) isolates were fosfomycin-susceptible, and the remaining 121 (75.1%) were resistant. Conclusion: Our study showed that using fosfomycin disc diffusion criteria of E. coli for other organisms is not ideal; there- fore, performing an alternative form of susceptibility testing for non-E. coli isolates is recommended. Continuous monitoring of fosfomycin susceptibility is warranted to detect any emerging resistance and to guide its clinical application.