O0842 Development of a panel for rapid susceptibility testing towards seven relevant antibiotics using MALDI-TOF MS-based direct-on-target microdroplet growth assay (DOT-MGA) Ilka Davina Nix*1, Evgeny A. Idelevich1, Katrin Sparbier2, Oliver Drews2, Markus Kostrzewa2, Karsten Becker1 1 Institute of Medical Microbiology, University Hospital Münster, Münster, Germany, 2 Bruker Daltonik GmbH, Bremen, Germany Background: A novel MALDI-TOF mass spectrometry (MS)-based direct-on-target microdroplet growth assay (DOT-MGA) was recently established for rapid antibiotic susceptibility testing of bacteria. In this study, we investigated a panel of seven antibiotics applicable for the treatment of infections caused by Enterobacterales using the MALDI-TOF MS- based DOT-MGA. Materials/methods: Twelve Enterobacteriaceae isolates comprising Escherichia coli, Klebsiella pneumoniae, Enterobacter spp. and Citrobacter spp. (each n = 3) and three Hafniaceae (Hafnia alvei) consecutive clinical isolates were included. A therapeutically relevant panel of seven antibiotics with anti-Enterobacterales activity including piperacillin, piperacillin/tazobactam, cefotaxime, ceftazidime, ciprofloxacin, gentamicin and meropenem was tested by DOT- MGA at EUCAST breakpoint concentrations to allow categorisation as either susceptible or resistant isolate. Bacterial suspensions (5x105 cfu/ml) with and without antibiotics in cation-adjusted Mueller-Hinton broth were placed onto an MBT Biotarget 96 (Bruker Daltonik) as 6-μl microdroplets. Two spotting replicates of each antibiotic as well as the growth control were prepared. The inoculated targets were incubated in a humidity chamber at 36°C for 4.5 hours. Subsequently, medium was removed using novel absorptive pads (Bruker Daltonik) and matrix was spotted onto the dried spots. MALDI-TOF MS measurements were performed in duplicate and acquired spectra were analysed by a newly developed prototype software (Bruker Daltonik). Each isolate was tested independently three times and median results were calculated. Broth microdilution was used as reference method. Results: Applying the novel prototype software, comparison of the results obtained by the MALDI-TOF MS DOT-MGA with those obtained by the standard broth microdilution approach revealed a categorical agreement of 93.0% for all seven antibiotics. In contrast to 20 h incubation time necessary to perform the microdilution, MALDI-TOF MS DOT- MGA was ready for readout already after 4.5 h incubation. Overall, 2.0% of the tested isolates showed false- resistant results and 5.0% showed false-susceptible results. The validity of the approach, i.e. successful detection of growth control, was 96%. Conclusions: Testing a panel of seven antibiotics, this study indicated that simultaneous susceptibility testing of antibiotics by MALDI-TOF MS-based DOT-MGA is feasible and accurate within 4.5 hours. Further research is required to standardise and optimise this approach. 29TH ECCMID 13-16 APRIL 2019 AMSTERDAM, NETHERLANDS POWERED BY M-ANAGE.COM .