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Chih-Cheng Lai,1 Ying-Sheng Chen,2 Nan-Yao Lee,3 Hung-Jen Tang,4,5 Susan Shin-Jung Lee,6,7 Chin-Fu Lin,8 Po-Liang Lu,9–11 Jiunn-Jong Wu,12 Wen-Chien Ko,13 Wen-Sen Lee,14 Po-Ren Hsueh15,16 1Department of Intensive Care Medicine, Chi Mei Medical Center, Liuying, Taiwan; 2Division of Infectious Diseases, Department of Internal Medicine, Cardinal Tien Hospital, New Taipei City, Taiwan; 3Department of Internal Medicine, National Cheng Kung University Medical College and Hospital, Tainan, Taiwan; 4Department of Internal Medicine, Chi Mei Medical Center, Tainan, Taiwan; 5Department of Health and Nutrition, Chia Nan University of Pharmacy and Science, Tainan, Taiwan; 6Department of Internal Medicine, Kaohsiung Veterans General Hospital, Kaohsiung, Taiwan; 7Faculty of Medicine, School of Medicine, National Yang Ming University, Taipei, Taiwan; 8Department of Pathology and Laboratory Medicine, Taichung Veterans General Hospital, Taichung, Taiwan; 9Department of Internal Medicine, Kaohsiung Medical University Hospital, College of Medicine, Kaohsiung Medical University, Kaohsiung, Taiwan; 10Department of Laboratory Medicine, Kaohsiung Medical University Hospital, Kaohsiung, Taiwan; 11Graduate Institute of Medicine, College of Medicine, Kaohsiung Medical University, Kaohsiung, Taiwan; 12Department of Biotechnology and Laboratory Science in Medicine, National Yang-Ming University, Taipei, Taiwan; 13Department of Medicine, College of Medicine, National Cheng Kung University, Tainan, Taiwan; 14Division of Infectious Diseases, Department of Internal Medicine, Wan Fang Medical Center, Taipei Medical University, Taipei, Taiwan; 15Department of Laboratory Medicine, National Taiwan University Hospital, National Taiwan University College of Medicine, Taipei, Taiwan; 16Department of Internal Medicine, National Taiwan University Hospital, National Taiwan University College of Medicine, Taipei, Taiwan Objectives: This study aimed to determine the in vitro susceptibility of commonly encountered Gram-negative bacilli (GNB) recovered from patients admitted to intensive care units (ICUs) in Taiwan against colistin, carbapenems, and other comparative agents. Methods: In total, 758 nonduplicate GNB isolates were obtained from clinical specimens of ICU patients at seven medical centers in 2016. Minimum inhibitory concentrations (MICs) were determined using the Vitek 2 susceptibility system. The reference broth-microdilution method was performed to determine MICs of colistin. Five main carbapenemase genes among carbapenem-non-susceptible GNB and mcr-1–mcr5 genes among colistin non-wild-type or -resistant isolates were determined. Results: After exclusion 38 Proteus mirabilis and 13 Morganella morganii spp. among 361 Enterobacteriaceae isolates, 34 (9.4%) isolates were carbapenem-insusceptible, 91.1% (n=31) were colistin wild type, and three and one Klebsiella pneumoniae isolates carried blaKPC and blaOXA48-like, respectively. Carbapenem-insusceptible isolates were found in 23.4% (30 of 128) and 63.0% (87 of 138) of isolates of the Pseudomonas aeruginosa and Acinetobacter baumannii complex, respectively. mcr-1 was detected in two (1.8%) Enterobacter cloacae isolates. Very major errors between two methods of susceptibility to colistin were found in 1.5% of K. pneumoniae, 27.5% of E. cloacae, 4.7% of P. aeruginosa, and 10.1% of A. baumannii complex isolates. Conclusion: In this study, 8.7% of Enterobacteriaceae isolates from ICUs were not susceptible to carbapenem, and blaKPC and blaOXA48-like were found among three and one carbapenem-insusceptible K. pneumoniae isolates, respectively. Colistin MICs determined by Vitek 2 were not reliable, especially for E. cloacae and A. baumannii complex isolates. Keywords: colistin, carbapenems, susceptibility, carbapenemase, mcr-1, intensive care units, SMART, P. aeruginosa, A. baumannii