Investigation for the Colistin Resistance Genes mcr-1 and mcr-2 in Clinical Enterobacteriaceae Isolates from Cats and Dogs in Switzerland

Colistin resistant Enterobacteriaceae have emerged in humans, food producing animals and food. The aim of this study was to assess the occurrence of MCR-1 and MCR-2 producers among Enterobacteriaceae isolated from companion animals admitted to the University of Zürich veterinary clinic. A total of 347 isolates (231 from dogs, 116 from cats) were analysed. Thereof, 274 (79%) were from urine and 73 (21%) from surgical sites, abscesses and other sources. All isolates were screened by polymerase chain reaction (PCR) for the presence of the plasmid-mediated colistin resistance genes mcr-1and mcr-2. Phenotypic colistin resistance was screened for by the antibiotic broth microdilution method. None of the isolates tested positive by PCR for the mcr-1 or the mcr-2 gene. Broth microdilution tests revealed MICs of 2mg/L–64 mg/L of colistin for four isolates (two Enterobacter spp., one Klebsiella spp. and one E. coli) from dogs (one from skin exudate, one from a surgical site and two isolates from urine). This study documents the presence of non-transmissible colistin resistance in clinical isolates from dogs and cats and confirms that so far, companion animals in Switzerland do not represent a reservoir for plasmid-mediated colistin resistant Enterobacteriaceae.