The researchers found the gene in pig-associated clinically relevant Salmonella serotypes and clones recovered from human clinical samples and pork, collected as early as 2011.
The mcr-1 gene makes bacteria resistant to the antibiotic colistin, which is used as a last-resort drug to treat patients with multi-drug-resistant infections, including carbapenem-resistant Enterobacteriaceae (CRE).
It exists on a plasmid, a small piece of DNA that is capable of moving from one bacterium to another, spreading antibiotic resistance among bacterial species.
MCR-1 gene found in isolates
The MCR-1 gene was first reported in China towards the end of 2015.
Researchers analysed 1,010 Salmonella isolates of 58 serotypes from human clinical cases, food products, food-animal production settings and aquatic environments and regions of Portugal, collected between 2002 and 2015.
Isolates positive for mcr-1 by PCR were tested for susceptibility to colistin by the broth microdilution method and interpreted according to the European Committee on Antimicrobial Susceptibility Testing.
The mcr-1 gene was detected in 11 (1.1%) of the Portuguese Salmonella isolates, recovered from human clinical sources and pork food products across the country.
This gene had 100% homology with the first published mcr-1 sequence in an E. coli strain from China, they said.
Serotypes of positive isolates
The 11 mcr-1-positive Salmonella isolates belonged to the serotypes S. 1,4,[5],12:i:- and S. Rissen.
In both cases, the team found them associated with particular successful multidrug-resistant (MDR) clonal lineages, either of the S. Rissen/ST469 clone or the S. 1,4,[5],12:i:-/ST34 European clone that is currently spreading in European countries.
During the study period, Salmonella isolates harbouring the mcr-1 gene were only recovered between 2011 and 2015 and originated from human clinical sources (4/522) and pork products, mostly from slaughterhouses (7/296).
Data from 2004 to 2006 had shown high use of Colistin for food-producing animals in Portugal, which is one of the European countries with highest consumption of polymyxins that has been increasing in the last years (2011–13).
“Taking into account the current picture of colistin use in Portugal, the detection of mcr-1 in the most recent collections and in pork products is of concern. Nevertheless, data on chronology, current prevalence of the mcr-1 gene and its evolution in bacteria from animals, food and humans are lacking,” said the researchers.
Source: Eurosurveillance, Volume 21, Issue 26, 30 June 2016
“Mcr-1 multidrug-resistant and copper-tolerant clinically relevant Salmonella 1,4,[5],12:i:- and S. Rissen clones in Portugal, 2011 to 2015”
Authors: J Campos, L Cristino, L Peixe, P Antunes