Polymyxin resistance and mcr-1 prevalence in non-typhoid Salmonella isolates in China

Objective To investigate the prevalence of polymyxin (polymyxin B and polymyxin E) resistance in non-typhoid Salmonella strains in China. Methods Phenotypic antimicrobial susceptibility test was conducted by means of broth dilution to detect the prevalence of polymyxin resistance in 404 strains of non-typhoid Salmonella from different sources in China. The resistant breakpoints for polymyxin B and polymyxin E were set based on minimal inhibitory concentration (MIC) distribution data. The carriage of mcr-1 gene in polymyxin resistant Salmonella strains was detected with PCR. Results The polymyxin MICs ranged from 0.125 g/ml to 16 g/ml. The MIC50 and MIC90 of polymyxin B were 1 g/ml and 8 g/ml, respectively. While the MIC50 and MIC90 of polymyxin E were 2 g/ml and 8 g/ml, respectively. The distributions of MICs for different predominant serotypes, including Salmonella Typhimurium, Salmonella Enteritidis and Salmonella Derby, were different. The resistant breakpoint MICs for polymyxin B and polymyxin E were set at 8 g/ml based on their MIC frequency distribution. According to this criteria, the overall resistance rates to polymyxin B and polymyxin E were 10.89% and 15.84%, respectively. The resistance rates of Salmonella with different sources to polymyxin B differed significantly (food source 12.50%, human source 17.16% and animal source 0) (P0.01), so did the resistance rates of Salmonella with different sources to polymyxin E (food source 8.30%, human source 27.94% and animal source 0.78%) (P0.01). Importantly, seven Salmonella strains showed co-resistance to polymyxin and the third-generation cephalosporins. Moreover, one Salmonella Typhimurium strain producing ESBLs isolated from human stool was mcr-1 gene positive and this gene was carried by a transferable plasmid. Conclusion This is the first report to identify a human original Salmonella strain carrying the mcr-1 gene. Based on MIC distribution data in vitro, MIC of 8 g/ml was recommended as the resistant breakpoints to polymyxin B and colistin for non-typhoid Salmonella in China. The prevalence of polymyxin resistance was low, but it is not negligible. The antimicrobial resistance surveillance for Salmonella from different sources should be strengthened.