Epidemiology of infections caused by polymyxin-resistant pathogens

- Chromosomally-mediated colistin resistance in Gram-negatives identified since 2004.
- Highest predominance of chromosomally-mediated resistance in K. pneumoniae mainly from Greece, Italy and Romania.
- Emergence of plasmid-mediated colistin resistance due to mcr-1 gene recently observed in China.
- As of 1 September 2016, mcr-1 was detected worldwide from livestock/retail meat and human sources from 35 countries.

Confronting the storm of carbapenemase-producing Gram-negative pathogens and thus facing the threat of untreatable infections, the medical community revived colistin. Not long since its re-introduction and despite the fact that resistance to colistin at least in Escherichia coli is rare, chromosomally-mediated colistin resistance in metallo-β-lactamase-producing Klebsiella pneumoniae strains was reported in 2004 from Greece. Subsequent studies revealed the highest predominance in Italy (38%) and Greece (26%), with colistin-resistant (Col-R) strains frequently carrying a carbapenemase. On the other hand, the international prevalence of Col-R Acinetobacter baumannii varied, predominantly in Southern Europe and Southeast Asia, with rates exceeding 80% in Italy and Greece. Risk factors have mainly incriminated the selective pressure of excess consumption of colistin both in animals and humans. In November 2015, emergence of plasmid-mediated colistin resistance due to the mcr-1 gene was reported from China, mostly in community-derived E. coli strains. As of 1 September 2016, the mcr-1 gene was detected in 35 countries worldwide in livestock/retail meat and in human sources from 29 and 22 countries, respectively. Heavy usage of polymyxins in animals has been incriminated as the reservoir of the mcr-1 gene. Therefore, it is imperative that: (i) polymyxins are banned as growth promoters and for prophylaxis in animals; (ii) targeted surveillance plus molecular epidemiology is performed in hospitals; (iii) carriers or patients infected with isolates harbouring both mcr-1 and carbapenemase genes are strictly isolated; (iv) susceptibilities are based on exact colistin minimum inhibitory concentration (MIC) determination; and (v) rational use of colistin is audited in hospitals.