Molecular Evidence for Intra- and Inter-Farm Spread of Porcine mcr-1-Carrying Escherichia coli in Taiwan

From January 2013 to December 2018, 90 Escherichia coli isolates were collected from various sites in 90 sick pigs on 58 farms located in 7 cities in Taiwan. The minimum inhibitory concentrations (MICs) of these isolates to 14 antimicrobial agents were determined using the Vitek 2 system (bioMerieux, Marcy-l’Etoile, France) and to colistin using the reference broth microdilution (BMD) method. The mobilized colistin resistance genes (mcr) were determined for colistin-resistant isolates, which displayed BMD MICs 4 mg/L. Genotypes of mcr-positive E. coli isolates were determined by multilocus sequence typing, arbitrarily primed polymerase chain reaction (PCR), and pulsed-field gel electrophoresis. All the isolates were susceptible to the carbapenems tested. Fifty isolates (55.6%) were resistant to colistin. Of these, 39 (78%) were positive for mcr-1 gene. E. coli isolates harboring mcr-1 were obtained most frequently in 2017 (15/18, 83.3%), followed by 2018 (13/23, 56.5%), 2015 (7/21, 33.3%), and 2016 (3/24, 12.5%). Among the 39 porcine mcr-1-carrying E. coli isolates, 18 sequence types (STs) were identified. Of these, 13 (33.3%) were ST2512, and all 13 were isolated in 2017 and 2018. A total of five genotypes (clones) isolates were identified. Isolates of the same genotypes were identified in different sick pigs on the same farms as well as on different farms, indicating the intra- and inter-farm spread of porcine mcr-1-carrying E. coli. The data indicate the high prevalence of colistin resistance and presence of mcr-1 in E. coli isolates among sick pigs in 2015-2018 in different regions of Taiwan.