Objective:

Non-typhoidal Salmonella (NTS) is a major cause of human salmonellosis globally and food animals are major reservoirs. The rising trend of antimicrobial resistance (AMR) of foodborne NTS led to clinical treatment failure. We tested the antimicrobial susceptibility of 1,256 NTS cultured from retail foods in 2020 in China to better understand the prevalence and characterization of AMR foodborne NTS of China. 

Method:

The antimicrobial susceptibility of 26 antimicrobial agents representing 12 classes was evaluated by the broth-microdilution; the presence of ten mcr genes was screened by multi-PCR. The complete closed genomes of mcr-gene-carrying isolates were obtained by whole genome sequencing using both PacBio and Illumina platforms. Genomic features and genetic environments of mcr-1 gene were analysed.

Results:

The whole drug resistance rate was 92.28% and the multiple drug resistance (MDR) rate was 76.53%. Resistance was the highest to nalidixic acid (63.38%); 341 AMR profiles were recorded. Among 887 MDR NTS, 232 showed co-resistance to cefotaxime and ciprofloxacin, and 25 were resistant to ten classes of antimicrobials. The resistance of NTS isolated from different regions varied. Isolates of raw chicken source showed most frequent resistance. Four NTS carried mcr-1 gene and they represented four different serotypes. Four mcr-1 gene-harboring plasmids from the four Salmonella isolates were denoted as two replicon types (IncI2 and IncHI2A). Two mcr-1 genes on IncI2 type plasmids were found to be located between a PAP2 family protein-encoding gene and a relaxase-encoding gene, while the other two mcr-1 gene structures located at IncHI2A type plasmids varied with different presence of insertion sequences.

Conclusion:

Our data demonstrated a severe AMR for foodborne NTS isolated from food in China, and highlighted the importance of antimicrobial susceptibility surveillance to reduce resistance dissemination, especially to the concerned critically important drugs for human medicine.