647. Investigation of Heteroresistance Among Klebsiella pneumoniae (KP) Inner Colonies (IC) Observed During Fosfomycin Disk Diffusion (DD) Testing

Background During fosfomycin DD testing, the frequent occurrence of non-susceptible IC within the zone of inhibition of susceptible isolates has been noted. The Clinical & Laboratory Standards Institute (CLSI) and European Committee on Antimicrobial Susceptibility Testing (EUCAST) have contradicting recommendations on how IC should be interpreted; CLSI recommends considering IC when interpreting DD results whereas EUCAST recommends ignoring them. This study sought to identify the susceptibility of these IC and to understand whether heteroresistance contributes to the appearance of IC during fosfomycin DD. Methods This study included a convenience sample of 71 KP clinical isolates from 3 United States locations. During DD testing, 58 (81.7%) of these isolates displayed at least one IC. Broth microdilution (BMD) minimal inhibitory concentration (MIC) testing, using extrapolated CLSI Escherichia coli breakpoints, was performed on a subset (n=32) of the IC in duplicate for comparison to the corresponding parent MIC values. This was followed by a modified disk elution screening test for heteroresistance to compare the frequency of low level resistance (LLR) and high level resistance (HLR) between the susceptible isolates that produced resistant IC (n=6) and those that did not produce any IC (n=3). Results The MIC range for the IC isolates (128 to > 1024 μg/mL) increased as compared to the parent isolates (< 2 to > 256 μg/mL) and MIC50/90 increased from the parent (128/ > 256 μg/mL) to IC (1024/ > 1024 μg/mL) isolates. All IC isolates had a resistant MIC value vs. 46.5% of parent isolates, and over 90% of IC isolates had an MIC at least 2 dilutions higher than their corresponding parent isolate. Heteroresistance screening found all tested isolates to be positive for LLR, and 8 of 9 positive for HLR, while the one HLR-negative isolate was IC-producing. Conclusion IC were frequent during fosfomycin DD testing and were commonly more resistant than their corresponding KP parent isolates. A small subset of these isolates tested via a modified disk elution test displayed either LLR or HLR regardless of the absence of IC. These results call for further investigation among a larger isolate set to understand what mechanisms are responsible for the frequency of IC and their increased fosfomycin resistance. Disclosures All Authors: No reported disclosures