Emergence of mcr- 1 Producing Salmonella enterica serovar Typhimurium from Retail Meat: First Detection in Brazil

Dear Editor:

Resistance to polymyxins is of great concern to public health as these are the last resource for treatment of infections caused by carbapenemase-producing Enterobacteriaceae. Polymyxin resistance determinants were restricted to chromosomal mutations until 2015, when a plasmid carrying the mcr-1 gene was described (Liu et al., 2016). The mcr-1 gene was detected worldwide in Escherichia coli and less often in other Enterobacteriaceae species, including Salmonella sp. (Poirel et al., 2017). The mcr-1 gene in Salmonella was first described in Europe (Petrillo et al., 2016) and later in China (Yang et al., 2016). Recently, the Pan American Health Organization published an epidemiological alert describing colistin resistant mcr-1 Salmonella enterica serovar Typhimurium, isolated from patients, in Colombia (PAHO/WHO, 2016). In Brazil the mcr-1 was detected first in E. coli isolates, from diverse origin (Fernandes et al., 2016; Lentz et al., 2016; Dalmolin et al., 2017; Sellera et al., 2017) and recently in Klebsiella pneumoniae (Aires et al., 2017).

In view of the concerning spread of mcr-1 gene among food-producing animals, we screened, by polymerase chain reaction as previously described (Liu et al., 2016), 40 Salmonella isolates, obtained from animal products between 2011 and 2017. One isolate, collected from a retail frozen pork in 2016, at a commercial establishment, in southern Brazil, was positive for mcr-1 and was named Salmonella Typhimurium SLRe1.

Conjugation experiments confirmed the transferable origin of the mcr-1. The minimal inhibitory concentration (MIC) of several antibiotics was evaluated by broth microdilution for the Salmonella Typhimurium SLRe1 and its transconjugant (Table 1). The transconjugant presented the same MIC for colistin (8 mg/L) as Salmonella Typhimurium SLRe1. Although the isolate Salmonella Typhimurium SLRe1 is susceptible to cephalosporins and meropenem, it is resistant to clinical important antibiotics such as chloramphenicol, ciprofloxacin, and tetracycline. Nevertheless, colistin resistance was the only resistance marker which was transferred by conjugation, showing that the plasmid carrying mcr-1 confers resistance only to colistin. Other plasmids were present in the clinical isolate, but these were not selected in the conjugation experiment performed in this study.

Brazil is the fourth largest pork producer in the world and exports to more than 70 countries. In this context, Polymyxins, which were commonly used as growth promoters in animal production, are forbidden in Brazil since November 2016. For this reason, a regular random sampling should be implemented to monitor the presence of the mcr-1 gene in retail meat, after polymyxin prohibition. To the best of our knowledge, this is the first report of mcr-1 in Salmonella Typhimurium in Brazil, highlighting the spread of this gene, threatening the use of polymyxins as a last resort antibiotic in clinical practice.