Multidrug-resistance and mobile colistin resistance (mcr) genes of Salmonella isolates from pork in Thailand during 2014-2017: comparison between two different types of slaughterhouses and retails

Food-producing animals are the major reservoir for Salmonella infections in humans. Salmonella contamination and spread of antimicrobial resistance genes can occur during the production chain of animal products. The aims of this study were to investigate antimicrobial resistance patterns and compare the proportions of multidrug resistance and the presence of mobile colistin resistance (mcr) genes, mcr-1, mcr-2 and mcr-3, among Salmonella isolates which were recovered from pork at two different standard practice slaughterhouses and retails during 2014-2017 in Thailand. Salmonella isolates recovered from good standard practice slaughterhouses (GSH, n=75), below standard practice slaughterhouses (BSH, n=75), good standard practice retails (GRT, n=75) and below standard practice retails (BRT, n=75) were examined for their antimicrobial resistance patterns and the existence of mcr-1 to mcr-3 genes. Salmonella strains of the 4 origins showed similar resistance rates to almost all antimicrobial agents tested. BRT origin (33/75, 44%) had slightly higher proportion of MDR Salmonella than the others group with no statistical difference. Five MDR Salmonella isolates carrying the mcr-3 gene were detected among isolates of all origins, while only 4 isolates (1.33%) displayed colistin resistance phenotype (MIC 4-8 ug/mL). This study revealed that MDR Salmonella isolates have widely spread in both standard and low hygiene practice slaughterhouses and retails. This is the first report of mcr-3 positive MDR Salmonella isolates from pork in Thailand. Effective monitoring program in slaughterhouses and retails should be continually implemented to reduce the contamination of MDR Salmonella carrying the mcr gene to consumers.