In Vitro Susceptibility of Common Urinary Tract Pathogens to Fosfomycin
Urinary tract infections (UTI) are common in both outpatient and inpatient settings. Urinary pathogens are increasingly resistant to many commonly used antimicrobial agents. Fosfomycin is an approved agent for UTI, however, there are few recent large-scale susceptibility surveys. Here we report a prospective survey of in vitro susceptibility of urinary isolates to fosfomycin at a tertiary care institution.
Bacterial strains isolated from urine from September – December, 2013 were collected from the clinical laboratory at the Jackson VA Medical Center. Isolates were tested using Vitek ®2 system and specifically with fosfomycin using E test according to CLSI protocol. MICs were interpreted according to CLSI breakpoints where such standards are published, otherwise, MICs are reported in mg/mL.
A total of 198 unique isolates were included: Enterobacteriaceae - 132, Pseudomonas aeruginosa - 22, Enterococcus spp. - 22, Acinetobacter baumanii - 3, Stenotrophomonas maltophila - 1, Staphylococcus spp. – 9, ESBL - 9. CLSI breakpoints for fosfomycin are established only for E. coli and E. faecalis. For these, 98% of E. coli and 94% of E. faecalis isolates were susceptible. For ESBL- producers eight were E.coli and one was Klebsiella pneumonia. Only one ESBL E. coli was resistant to fosfomycin. MIC50 and MIC90 for Enterobacteriaceae isolates were 4 and 96 respectively. MIC50 and MIC90 for P. aeruginosa isolates were 128 and >1024 respectively. Interestingly, of eight unique isolates of Morganella morganii, all demonstrated resistance to fosfomycin, with MICs >1024. Glucose non-fermentative organisms in this study (Acinetobacter, Stenotrophomonas) had MICs at or above 96 mg/mL.
Fosfomycin is an effective yet under-utilized treatment for UTIs caused by Enterobacteriaceae, including ESBL-producers. An exception to this is M. morganii, which appears to be intrinsically resistant to this agent. Further large surveys are needed to confirm this observation. Fosfomycin appears to be highly active against the two most common Enterococcus species found in UTIs in humans. The activity of fosfomycin against Pseudomonas is not predictable and use of fosfomycin to treat UTIs with this pathogen should be guided by in vitro testing.
D. Chastain, None
A. Swiatlo, None