Carriage of colistin-resistant, extended-spectrum beta-lactamase-producing Escherichia coli harboring the mcr-1 resistance gene after short-term international travel to Vietnam Tatsuya Nakayama,1,2 Yuko Kumeda,3 Ryuji Kawahara,4 Takahiro Yamaguchi,4,5 Yoshimasa Yamamoto5 1Center for Global Initiatives, Osaka University, Suita, Osaka, Japan; 2Division of Biomedical Food Research, National Institute of Health Science, Kawasaki, Kanagawa, Japan; 3Center for Microorganism Control, Osaka Prefecture University, Sakai, Osaka, Japan; 4Division of Microbiology, Osaka Institute of Public Health, Osaka, Japan; 5Graduate School of Pharmaceutical Sciences, Osaka University, Suita, Osaka, Japan Background: Due to increasing colistin usage, the dissemination of the colistin-resistant gene mcr-1 has been increasingly investigated. The aim of this study was to determine whether a traveler on a short-term international trip to a developing country could bring mcr-1 back to their home country. Materials and methods: Thirty-four travel events from Japan to Vietnam encompassing 19 travelers were assessed. A fecal specimen was collected from each traveler before and after each travel event and was inoculated on CHROMagar containing cefotaxime (CTX). Three to seven colonies exhibiting the characteristics of Escherichia coli were collected. Susceptibility to antibiotics and extended-spectrum β-lactamase (ESBL) production were determined by the disk diffusion method and the double-disk synergy test, respectively. ESBL-encoding genes were genotyped, and phylogenetic groupings were determined by multiplex polymerase chain reaction (PCR). The presence of mcr-1 was also confirmed by PCR and sequencing. Results: A total of 175 ESBL-producing E. coli isolated before and up to 2 weeks after traveling to Vietnam were analyzed. Genotyping of ESBL-producing isolates showed that blaCTX-M-1/blaTEM (27.7%) and blaCTX-M-9 (45.9%) were the most prevalent genotypes, while the most frequently detected phylogenetic group was D (41.9%) followed by B2 (23.0%). In a significant number of travel events, travelers brought ESBL-producing E. coliback to Japan and three events by three travelers carried mcr-1. ESBL-producing E. coliisolates harboring mcr-1 were identified as those carrying both blaCTX-M-14 or blaCTX-M-55 and mcr-1. Conclusion: Using Vietnam as an example, we have shown that even a short-term trip to some countries may result in ESBL-producingmcr-1-positive E. coli carriage by international travelers. Keywords: traveler, ESBL-producing E. coli, mcr-1, Vietnam, Japan