A review on the current situation and challenges of colistin resistance in poultry production.

Colistin has been re-assessed as a critically important antimicrobial in humans due to its efficacy against multi-drug resistant Gram-negative bacteria, in particular P. aeruginosa, A. baumannii, and K. pneumoniae. The recent discovery of mobile colistin resistance (mcr) determinants in humans and animals has brought concerns regarding the future of this antimicrobial. In this paper, we aim to highlight the current challenges with colistin resistant bacteria and to summarize reliable global data on colistin resistance in poultry production. In addition, we present and compare data from a screening for colistin resistance carried out on a collection of clinical Escherichia coli isolated from poultry in Italy. In Europe, resistance rates for Salmonella and E. coli are in general low with sporadic incidence of high colistin resistance levels. Absence of resistance or very low rates have been recorded in countries where colistin is either not employed (e.g. Norway) or used in minimal amounts (e.g. Denmark) in food-producing animals. In large poultry meat producing countries, such as China and Brazil, the widespread use of colistin has resulted in the dissemination of resistance determinants in diverse bacterial species. Worryingly, these bacteria are often co-resistant to other critically important antimicrobials, such as extended-spectrum cephalosporins. The data gap for many countries and for zoonotic bacteria, the role of the "phantom resistome" and the circulation of mcr-carriers expressing resistance phenotypes close or below the current ECOFF values, should be considered in future investigations. The importance of poultry as a cheap protein source and the global effort to mitigate colistin resistance and preserve this essential antimicrobial require a thorough re-assessment of colistin use in poultry.