Colistin is widely used as an antibiotic of last resort for treating infections caused by multidrug-resistant gram-negative bacteria such as carbapenemase-producing Enterobacteriaceae. Recently, the emergence of plasmid-mediated (horizontally-transferable) colistin resistance (mcr-1) has become a great challenge to global public health. The mcr-1 gene was detected in ESBL (Extended Spectrum β-Lactamase)- producing and/or carbapenemase-producing Enterobacteriaceae. Therefore, there is a huge risk of the emergence of pan-drug-resistant gram-negative bacteria. In this paper we discuss the epidemiological analyses of mcr-1 positive Enterobacteriaceae and structural analyses of PmrC that was recently identified as a protein associated with colistin resistance.