Multidrug resistance inEscherichia colihas become a worrying issue that is increasingly observed in human but also in veterinary medicine worldwide.E. coliis intrinsically susceptible to almost all clinically relevant antimicrobial agents, but this bacterial species has a great capacity to accumulate resistance genes, mostly through horizontal gene transfer. The most problematic mechanisms inE. colicorrespond to the acquisition of genes coding for extended-spectrum β-lactamases (conferring resistance to broad-spectrum cephalosporins), carbapenemases (conferring resistance to carbapenems), 16S rRNA methylases (conferring pan-resistance to aminoglycosides), plasmid-mediated quinolone resistance (PMQR) genes (conferring resistance to [fluoro]quinolones), andmcrgenes (conferring resistance to polymyxins). Although the spread of carbapenemase genes has been mainly recognized in the human sector but poorly recognized in animals, colistin resistance inE. coliseems rather to be related to the use of colistin in veterinary medicine on a global scale. For the other resistance traits, their cross-transfer between the human and animal sectors still remains controversial even though genomic investigations indicate that extended-spectrum β-lactamase producers encountered in animals are distinct from those affecting humans. In addition,E. coliof animal origin often also show resistances to other—mostly older—antimicrobial agents, including tetracyclines, phenicols, sulfonamides, trimethoprim, and fosfomycin. Plasmids, especially multiresistance plasmids, but also other mobile genetic elements, such as transposons and gene cassettes in class 1 and class 2 integrons, seem to play a major role in the dissemination of resistance genes. Of note, coselection and persistence of resistances to critically important antimicrobial agents in human medicine also occurs through the massive use of antimicrobial agents in veterinary medicine, such as tetracyclines or sulfonamides, as long as all those determinants are located on the same genetic elements.

The plasmid-located mcr-9 gene, encoding a putative phosphoethanolamine transferase, was identified in a colistin-resistant human fecal Escherichia coli strain belonging to a very rare phylogroup, the D-ST69-O15:H6 clone. This MCR-9 protein shares 33% to 65% identity with the other plasmid-encoded MCR-type enzymes identified (MCR-1 to -8) that have been found as sources of acquired resistance to polymyxins in Enterobacteriaceae .

In July 2017, a patient presented colonization with a multidrug-resistant, carbapenemase (KPC-3)-producing Klebsiella pneumoniae isolate. A custom-made, lytic bacteriophage preparation was administered to the patient in December 2017, with subsequent eradication of the microorganism and without adverse effects.

A series of colistin-resistant Escherichia coli clinical isolates was recovered from hospitalized and community patients in South Africa. Seven clonally unrelated isolates harbored the mcr-1 gene located on different plasmid backbones. Two distinct plasmids were fully sequenced, and identical 2,600-bp-long DNA sequences defining a mcr-1 cassette were identified. Promoter sequences responsible for the expression of mcr-1 , deduced from the precise identification of the +1 transcription start site for mcr-1 , were characterized.