Resistance to polymyxins in Gram-negative organisms

- Colistin resistance is not so rare in Gram-negative isolates.
- Colistin resistance in Gram-negative bacteria is mostly due to acquired mutations in two-component systems.
- Acquired foreign DNA (mcr genes) is more prevalent in Escherichia coli clinical isolates.

Polymyxins have recently been re-introduced into the therapeutic arsenal to combat infections caused by multidrug-resistant Gram-negative bacteria. However, the emergence of strains resistant to these last-resort drugs is becoming a critical issue in a growing number of countries. Both intrinsic and transferable mechanisms of polymyxin resistance have been characterised. These mechanisms as well as the epidemiological data regarding four relevant bacterial pathogens (Escherichia coli, Klebsiella pneumoniae, Acinetobacter baumannii and Pseudomonas aeruginosa) are considered in this review. A special focus is made on plasmid-mediated resistance and the spread of mcr genes.