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


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