Abstract
The polymyxin antibiotic colistin has been used in decades for treatment and prevention of infectious diseases in livestock. Nowadays, it is even considered as last-line treatment option for severe human infections caused by multidrug- and carbapenem-resistant Gram-negative bacteria. Therefore, the discovery of plasmid-mediated mobile colistin resistance (mcr) genes raised major public health concern. The aim of our study was to analyze colistin-resistant Salmonella enterica strains from animals, food, feed and the environment collected at the National Reference Laboratory for Salmonella in Germany on the presence of mcr-1 to mcr-9 genes. Altogether 407 colistin-resistant (MIC >2 mg/L) Salmonella isolates received between 2011 and 2018 were selected and screened by PCR using a published mcr-1 to mcr-5 as well as a newly developed mcr-6 to mcr-9 multiplex PCR protocol. 254 of 407 (62.4%) isolates harbored either mcr-1 (n = 175), mcr-4 (n = 53), mcr-5 (n = 18) or mcr-1 and mcr-9 (n = 8). The number of mcr-positive isolates ranged from 19 (2017) to 64 (2012) per year. WGS revealed that none of our isolates harbored the mcr-9.1 gene. Instead, two novel mcr-9 variants were observed, which both were affected by frameshift mutations and are probably non-functional. The mcr-harboring isolates were mainly derived from animals (77.2%) or food (20.1%) and could be assigned to ten different Salmonella serovars. Many of the isolates were multidrug-resistant. Co-occurrence of mcr-1 and AmpC or ESBL genes was observed in eight isolates. Our findings suggest that mcr genes are widely spread among colistin-resistant Salmonella isolates from livestock and food in Germany. Potential transfer of mcr-harboring isolates along the food chain has to be considered critically.
Highlights
Non-typhoidal Salmonella (S.) enterica subsp. enterica are zoonotic pathogens which can cause severe human infections through the consumption of contaminated food
PCR grade water was used as negative control (NTC) and HyperLadderII (Bioline) was used as molecular weight marker (L)
For monitoring the spread of all known mcr genes in the food chain, the available mcr-1 to mcr-5 multiplex PCR was complemented with a novel mcr-6 to mcr-9 multiplex PCR assay
Summary
Non-typhoidal Salmonella (S.) enterica subsp. enterica are zoonotic pathogens which can cause severe human infections (salmonellosis) through the consumption of contaminated food. Enterica are zoonotic pathogens which can cause severe human infections (salmonellosis) through the consumption of contaminated food. Due to the high prevalence, it is thought that foodproducing animals are most likely the origin for mcr-1 mediated colistin resistance (Nordmann and Poirel, 2016; Poirel and Nordmann, 2016). These findings and the reclassification of colistin in human medicine led to a reassessment of colistin by the European Medicines Agency (EMA) suggesting to reduce the use of colistin in livestock production as much as possible (EMA, 2016)
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