With the increasing occurrence of carbapenem-resistant Gram-negative bacteria (due to the spread of plasmid-coded carbapenemases), the antibiotic colistin (or polymyxin B), discovered in the early 1950s, gained clinical importance again due to the lack of alternatives. This reserve antibiotic had not been used in human medicine for a long time because of serious side effects.

Resistance to colistin was described very soon. These were inducible and were based on point mutations in various genes (eg mgrB, phoPQ or crrB) and could therefore not be spread horizontally.

Liu et al. described an E. coli isolate at the end of 2015 which has a gene "mcr-1" located on plasmids. This codes for a phosphoethanolamine transferase and gives the carrier bacterium resistance to colistin.

Other mcr-1-positive bacteria were quickly found worldwide, especially in isolates from veterinary medicine and food, but also from human isolates or environmental samples.

Another worrying fact is that carbapenem-resistant isolates with mcr-1 genes have also been found, which are therefore almost untreatable. In addition, mcr-1 has already been described in other species besides E. coli, e.g. in Klebsiella pneumoniae or Enterobacter spec ..