Detection of anmcr-1-encoding plasmid mediating colistin resistance inSalmonella entericafrom retail meat in Portugal: Table 1.

Sir, Recently, Liu et al.1 published the first report of plasmid-mediated colistin resistance (pHNSHP45) in Escherichia coli and Klebsiella pneumoniae in retail meat and in patients observed in the period 2011–14. Diverse commentaries on this finding followed this publication. Previously, a surveillance study was performed in Portugal to evaluate the susceptibility to 33 antibiotics and associations of 258 Salmonella enterica strains isolated from animal food products, using a commercial microdilution broth method (Microscan Panel, Siemens, West Sacramento, CA, USA).2 Resistance to colistin was observed in 37 isolates (14%). These isolates have now been screened for the presence of mcr-1 by PCR1 and amplicons sequenced. Four S. enterica serovar Typhimurium strains were positive, having been isolated from diverse food products of swine, poultry or cattle origin obtained in 2011 and 2012. The antimicrobial susceptibility profile was different for each strain (Table 1). Strain Sal25 also produced the ESBL CTX-M-1, which was co-transferred with the mcr-1 gene. The mcr-1 gene was successfully transferred to the recipient E. coli J53 (resistant to sodium azide) from all four strains at a frequency of around 10−4 cells per recipient by conjugation. Plasmid identification by PCR revealed that the two strains Sal23 and Sal25 harboured (100 and 150 kb, respectively) IncHI2 plasmids, but it was not possible to identify by the Carattoli et al.3 scheme the plasmids in Sal90 and Sal397. This observation supports the diversity of genetic platforms of mcr-1 with the ability to disseminate. Origins of and MICs (mg/L) for S. enterica strains from Portuguese retail meat positive for mcr-1 AMC, amoxicillin/clavulanate; CLA, clavulanate; TZP, piperacillin/tazobactam; SXT, trimethoprim/sulfamethoxazole; R, resistant; I, intermediate; S, susceptible. aR/I/S according to the CLSI guidelines, 2010. bR/S according to EUCAST clinical breakpoints, 2014. Origins of and MICs (mg/L) for S. enterica strains from Portuguese retail meat positive for mcr-1 AMC, amoxicillin/clavulanate; CLA, clavulanate; TZP, piperacillin/tazobactam; SXT, trimethoprim/sulfamethoxazole; R, resistant; I, intermediate; S, susceptible. aR/I/S according to the CLSI guidelines, 2010. bR/S according to EUCAST clinical breakpoints, 2014. We do not know the exact geographical origin of the animals, and the meat products might include meat from diverse sources, or contamination may have occurred during food processing. However, this study indicates that the mcr-1 gene was already present beyond Asian frontiers in 2011. The IncHI2 plasmid differs from the Incl2-like plasmids found in Asian E. coli, and two of them did not have a known plasmid replicon type. Curiously, in Germany, mcr-1 was found in transferable IncHI2 plasmids from E. coli of swine origin.4 Therefore, plasmid-mediated colistin resistance might be more frequent in Europe than initially thought. From comparative genome analysis,5 the mcr-1 gene in S. enterica Typhimurium Sal25 (accession number PRJEB9808),6 was carried on DNA contig_22 showing high sequence identity with the ISApl1-clrA portion of the Chinese plasmid pHNSHP45.1 Several publications followed the first report of the emergence mcr-1,1 describing the gene in food animals and human patients, and associated with ESBLs, as we found in Portuguese Sal25.4 From the literature, it is obvious that mcr-1 is present in different plasmids and appears to have inserted into different genetic environments, factors that may impact its spread geographically. Hence more genome data should be generated to fully understand the threat of a rapid widespread distribution. Nevertheless, our findings, and those of others, call for immediate action at an international level on the use of colistin in animal production. This work was supported financially by the Grant SFRH/BD/78833/2011 from Fundação para a Ciência e Tecnologia, Lisboa, Portugal and by the UK government Department for Environment, Food and Rural Affairs. None to declare.