Widespread amphotericin B-resistant strains of Aspergillus fumigatus in Hamilton, Canada Eta E Ashu,1 Gregory A Korfanty,1 Himeshi Samarasinghe,1 Nicole Pum,1 Man You,1 Deborah Yamamura,2 Jianping Xu1 1Department of Biology, McMaster University, Hamilton, ON, Canada; 2Department of Pathology and Molecular Medicine, McMaster University, Hamilton, ON, Canada Purpose: Amphotericin B (AMB) is one of the major antifungal drugs used in the management of aspergillosis and is especially recommended for treating triazole-resistant strains of Aspergillus fumigatus. However, relatively little is known about the AMB susceptibility patterns of A. fumigatus in many parts of the world. This study aims to describe the AMB susceptibility patterns in Hamilton, Ontario, Canada. Methods: The in vitro susceptibilities of 195 environmental and clinical A. fumigatus isolates to AMB were tested by the broth microdilution method as per the Clinical and Laboratory Standards Institute’s guidelines. Catalase-generated oxygen bubbles trapped by Triton X-100 were used to quantify catalase activity in a representative group of isolates. Results: Of the 195 isolates, 188 (96.4%) had the minimum inhibitory concentration (MIC) of AMB ≥2 mg/L, with approximately 80% and 20% of all clinical and environmental isolates having MICs of ≥ 4 mg/L. Overall, the clinical isolates were less susceptible to AMB than environmental isolates (P-value