P2771 Mecillinam MIC distributions determinated by agar dilution in Enterobacterales other than Escherichia coli and Klebsiella spp. and correlation with disk diffusion Hanne Wiese Hallberg1, Martin Iversen1, Frank Hansen2, Erika Matuschek3, Gunnar Kahlmeter4, Barbara Juliane Holzknecht*1

1 University Hospital Copenhagen, Herlev, Dept. of Clinical Microbiology, Herlev, Denmark, 2 Statens Serum Institute, Department of Bacteria, Parasites and Fungi, Copenhagen S, Denmark, 3 Central Hospital, EUCAST Development Laboratory (EDL), Växjö, Sweden, 4 Central Hospital , EUCAST Development Laboratory (EDL), Växjö, Sweden Background: Mecillinam is a penicillin-derivate with activity against gram negative rods. In the Nordic countries, it is a first line agent for treatment of urinary tract infections (UTI). EUCAST provides an epidemiological cut-off value (ECOFF; 1 mg/L) for Escherichia coli and Klebsiella spp. and clinical breakpoints for E. coli, Klebsiella spp. and Proteus mirabilis for use in uncomplicated UTI (MIC breakpoint: S ≤ 8 mg/L). However, about 5 % of Enterobacterales isolated in UTI belong to other species. Thus, provision of minimal inhibitory concentration (MIC) distributions of these species to enable breakpoint settings is clinically relevant. The reference method for mecillinam MIC determination is agar dilution. The aim of the study was to determine the mecillinam MIC wild type distribution and correlation between MIC and disk diffusion inhibition zones in Enterobacterales other than E. coli and Klebsiella spp. Materials/methods: A collection of 511 Danish blood culture isolates belonging to the 11 most prevalent species of the order Enterobacterales (E. coli, Klebsiella pneumoniae, Klebsiella oxytoca, Proteus mirabilis, Proteus vulgaris, Citrobacter freundii complex, Citrobacter koseri, Enterobacter aerogenes, Enterobacter cloacae complex, Serratia marcescens, Morganella morganii) were included in the study. MIC was determined by agar dilution and disk diffusion testing was performed using EUCAST methodology.

Results: Mecillinam MIC and zone diameter distributions of 45 E. coli, 42 K. pneumoniae and 42 K. oxytoca corresponded to the distributions found in the EUCAST database. The 41 C. freundii, 49 C. koseri, 46 E. cloacae and 65 E. aerogenes isolates showed similar MIC distributions (figure) and good correlation between MIC and inhibition zone. For Proteus spp., M. morganii and S. marcescens, technical issues regarding MIC readings are currently investigated further. Figure: MIC distributions for Citrobacter spp. and Enterobacter spp. Conclusions: This study performed in one single laboratory indicates, that the ECOFF 1 mg/L also is valid for Enterobacterales other than E. coli and Klebsiella spp., at least for Citrobacter spp. and Enterobacter spp. Further studies, including more isolates and testing in different laboratories, need to be performed, before ECOFFs can be established.

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