Several antibiotics have been massively used at subinhibitory concentrations as growth promoters for more than 6 decades3 to improve weight gain and therefore, to maximize feed conversion efficiency in animal production. This use is currently under close scrutiny, as it may be – and for sure is – selecting and collaborating for multi-drug resistant enteric bacteria.

As a service to poultry producers we have been monitoring the susceptibility to antimicrobial agents of indicator bacteria isolated from intensive farming systems to support choosing in advance the most effective promoters and to reduce selection pressure. Escherichia coli is one of the selected species considered markers for antimicrobial resistance evolution.

From 2013 to date, we have obtained 304 E. coli isolates recovered from 129 broiler chicken farms (only poultry growers) located in several provinces of Argentina (Buenos Aires, Córdoba, Entre Rios, Rio Negro and Santa Fe). Isolates were recovered from fresh fecal samples collected randomly from clinically healthy chickens (aged 4–6 weeks). The analysis showed that almost half of them (49%) were found to be resistant to colistin, as determined by microdilution according to EUCAST breakpoint recommendations (version 6.0).

Colistin is considered a last-line antimicrobial agent retaining activity on multiresistant bacteria recovered from humans. Even when resistance was sporadically reported, it was assumed to be obtained by mutation of regulatory genes4. However, a gene conferring resistance to colistin was recently reported in conjugative plasmids2. This gene (mcr-1) was most frequently found in E. coli but also in other species such as Klebsiella pneumoniae and Salmonella1. Up to date, a large number of publications demonstrated its presence in isolates collected mainly from animal samples, and to a lesser extent, in samples of human origin.

After selecting approximately one third of our colistin-resistant E. coli, all of them were confirmed as mcr-1 producers by PCR and full gene sequencing. In Argentina, the mcr-1 determinant was previously reported in E. coli clinical strains isolated from inpatients5.

It should be noted that in our country, many producers have voluntarily stopped using colistin after the technical reports of these (our) results, even before the initial report of plasmid-borne transmission. In this regard, the World Health Organization recommends that the use of colistin be limited for the treatment of clinically affected animals. Moreover, in our country, official regulations governing the administration of these compounds in animal feed have already changed, including a gradual stepwise process with a final goal to completely ban their use by 2019.