Background
Fosfomycin treatment of urinary tract infections is increasingly attractive due to escalating antibiotic resistance rates among urinary pathogens. Standard antibiotic susceptibility testing
Conclusion
A better understanding of fosfomycin susceptibility and improved antibiotic susceptibility testing tools could improve diagnostic capability and clinical guidelines for fosfomycin treatment of urinary tract infections. What this study adds: This study highlights the importance of adhering to interpretive guidelines when performing antimicrobial susceptibility testing and the need for simplified, accurate and standardised susceptibility testing methodology and interpretation for fosfomycin in Enterobacterales organisms.
Methods
Forty-six each of E. coli and K. pneumoniae clinical isolates were obtained from a tertiary hospital in South Africa, from 01 June 2017 to 31 January 2018. Agar dilution, disk diffusion, and gradient diffusion were performed and interpreted using the Clinical Laboratory Standards Institute and European Committee on Antimicrobial Susceptibility Testing guidelines. Molecular resistance mechanisms were identified by whole genome sequence analysis.
Objective
We evaluated the performance of fosfomycin susceptibility testing in E. coli and Klebsiella pneumoniae to determine whether fosfomycin susceptibility is associated with molecular resistance mechanisms.
Results
Disk diffusion and gradient diffusion were accurate alternatives for fosfomycin susceptibility testing in E. coli (98% categorical agreement), but not in K. pneumoniae (47% categorical agreement). All E. coli isolates contained at least one resistance mechanism, but only one isolate with a fosA gene was resistant. In K. pneumoniae, 63% (29/46) and 70% (32/46) of isolates were susceptible to fosfomycin, using Clinical Laboratory Standards Institute and European Committee on Antimicrobial Susceptibility Testing breakpoints, respectively, despite all isolates containing a fosA gene and a uhpT mutation.