339. Feasibility of Heteroresistance Screening on Escherichia coli (EC) to Predict the Presence of Inner Colonies (IC) During Fosfomycin Disk Diffusion (DD) Testing

Abstract Background Growth of IC within the zone of inhibition during fosfomycin DD testing in EC are found, but CLSI and EUCAST interpretation criteria for IC are contradictory. There is a need to determine appropriate methods of interpretation to prevent resistant infections, thus evaluating the susceptibility of IC EC isolates in comparison to parent can help determine optimal methods for DD testing. A heteroresistance screening test has potential to identify isolates with IC. We sought to determine the feasibility of heteroresistance screening to predict the presence of IC in clinical EC isolates. Methods A convenience collection (n=48) of EC parent isolates underwent fosfomycin DD testing to identify those with IC. Broth microdilution (BMD) testing was performed on all parent and IC (n=34) to determine minimal inhibitory concentrations (MICs) to establish inclusion criteria for heteroresistance screening. A disk elution test for heteroresistance screening was performed in duplicate on separate days. In tubes of Mueller-Hinton broth, 6 commercial fosfomycin disks (each 200 μg fosfomycin and 50 μg glucose-6-phosphate) were eluted for 90 minutes. One hundred microliters of each bacterial isolate from an overnight culture was suspended in tubes. A positive test was a turbid tube after 72 hours of incubation. Results The parent isolates (n=48) had an MIC range of 1 to >256 μg/mL with a median of 4 μg/mL. The subset of IC isolates (n=34) had an MIC range between ≤8 and >1024 with a median of 128 μg/mL. Nineteen IC isolates met the inclusion criteria for the heteroresistance screening; 5 isolates did not produce IC (MIC range of 1-32 μg/mL) while 14 isolates produced IC (MIC range of 1-16 μg/mL). Of these 19 isolates, 6 (32%) were heteroresistant using the disk elution test. Of those with IC, 4 (29%) were heteroresistant while 2 (40%) isolates without IC were heteroresistant. Conclusion A heteroresistance screening test did not provide consistent data to predict the presence of IC among this EC collection. A larger isolate set and further studies are needed to understand the feasibility of a heteroresistance screening test and increased resistance in IC resulting from fosfomycin DD testing among EC. Disclosures Elizabeth B. Hirsch, PharmD, FCCP, FIDSA, Melinta: Advisor/Consultant|MeMed: Advisor/Consultant|Merck: Advisor/Consultant|Merck: Grant/Research Support.