335. Comparison of inner colony production among <i>Escherichia coli</i> (EC) and <i>Klebsiella pneumoniae</i> (KP) during fosfomycin disk diffusion (DD) testing

Abstract Background Recent studies have indicated the presence of IC arising within zone inhibition during fosfomycin DD testing. CLSI and EUCAST contradicting recommendations for interpreting these IC; recommends considering when results while ignoring them. Although testing is approved only use with EC per both organizations, it often used non-EC organisms despite neither organization publishing Enterobacterales breakpoints (BP). We sought to compare rates production among KP isolates in order assess applicability isolates. Methods A convenience sample 128 (80 48 EC) clinical varied phenotypic profiles from 3 United States locations were included. Of these, 26% (n = 21) 29% =14) displayed ESBL phenotypes 19% 15) KPC producing. Fosfomycin susceptibility via was conducted duplicate on separate days following interpretations recommended by each organization. Isolates considered IC-producing if at least 5 arose ≥ 1 replicate Results Overall, 61% 78) produced including 75% 36) 52.5% 42) (Table 1). that IC, 89% 33) susceptible 92% 34) oral IV BP. In contrast, 41% 17) 2% 1) The did not result significant differences diameters (1-14mm, EC; 2-13mm, KP) comparing procedures. Table 1.Production categorization based three sets BP This table compares collection different (CLSI, oral, IV) using entire subset (EC: 36/48 KP: 42/80). Conclusion categorized most isolates, those as susceptible. EUCAST’s larger measurements near identical CLSI. lower very few even IC. captures a more diverse consistent diversity measurements. These findings warrant further investigation into relevance KP. Disclosures Elizabeth B. Hirsch, PharmD, FCCP, FIDSA, Melinta: Advisor/Consultant|MeMed: Advisor/Consultant|Merck: Grant/Research Support.