2164. Activity of Fosfomycin (FOF) and Frequency of Nonsusceptible Inner Colonies During Susceptibility Testing of an International Collection of Clinical Pseudomonas aeruginosa (PA) Isolates

BACKGROUND: FOF has been used clinically for the treatment of PA infections in the absence of established interpretive criteria. A recent study identified a low frequency of nonsusceptible inner colony mutants during disk diffusion (DD) testing of Escherichia coli; however, the frequency of this phenomenon in PA isolates is not well characterized. We sought to determine FOF activity against an international collection of PA isolates and the frequency of inner colony mutants observed during Etest and DD testing. METHODS: Minimal inhibitory concentration (MIC) values were determined for a convenience collection of 109 PA ([70/94] 64.2% MDR) isolates from 4 institutions in the United States and Australia. MIC testing was conducted in duplicate on separate days utilizing agar dilution (AD), broth microdilution (BMD), DD, and Etest as recommended per Clinical and Laboratory Standards Institute (CLSI). CLSI E.coli interpretive criteria (≤ 64 mg/L susceptible) were used for MIC interpretations. The proportion of isolates containing inner colonies was determined using DD and Etest. Inner colony mutants were subcultured and retested using BMD with comparison to the parent isolate MICs. RESULTS: FOF MICs varied widely and ranged from 1024 mg/L with MIC(50)/MIC(90) values of 64/256 (AD), 64/512 (Etest), and 64/256 (BMD) mg/L. Using E. coli criteria, susceptible/resistant rates were: 60.5/17.4% for AD; 60.5/22.0% for Etest; 86.2/7.3% for DD; and 53.2/17.4% for BMD. Inner colonies were frequently observed in 38.5% and 35.8% of DD and Etest inhibition zones, respectively. After repeat testing, mutant MIC values ranged from 64 to > 1024 mg/L and a majority (85.9%) had MIC values ≥ 512 mg/L. CONCLUSION: Observed MIC values of this (64% MDR) collection varied widely with MIC(50/90) values commonly at or above the E. coli susceptibility breakpoint. Inner colony mutants were frequently observed and highly resistant. Whole-genome sequencing is currently underway for a subset of parent/mutant pairs to determine whether specific genetic alterations are attributed to the increased MICs. Based on these results, caution should be warranted in extrapolating E. coli breakpoints to other organisms, and treatment of PA with FOF should be further evaluated. DISCLOSURES: Elizabeth B. Hirsch, PharmD, Merck: Grant/Research Support, Research Grant; Nabriva Therapeutics: Advisory Board; Paratek Pharmaceuticals: Advisory Board.