Objective   To investigate the antibiotic resistance and molecular characteristics of Salmonella Typhimurium isolated from chicken and pork in Guangzhou, 2018, and provide data support for the rational use of antibiotics and the prevention of drug-resistant pathogens.

  Methods  The S. Typhimurium isolates were identified by using API 20E biochemical tests, and antibiotic susceptibility testing was performed by using Sensititre CMV3AGNF plates. The whole genome was sequenced by using Illumina Miseq platform to analyze sequence types (ST), antibiotic resistance genes (ARGs) and point mutations, and to construct phylogenetic tree.

  Results  The antibiotic resistance rates of S. Typhimurium strains isolated from chicken (n=15) and pork (n=21) in Guangzhou to ampicillin, nalidixic acid, tetracycline, chloramphenicol, and sulfisoxazole were ≥60.00%, and the multi-drug resistance (MDR) rates were more than 65.00%. The ampicillin-nalidixic acid-tetracycline-chloramphenicol-(trimethoprim/sulfamethoxazole-chloramphenicol)-gentamicin was the predominant antibiotic resistance pattern, and the drug resistance of S. Typhimurium isolated from pork was more serious. The carriage rates of tet(B), aph(3")-Ib, and aph(6)-Id were higher in pork isolates than in chicken isolates, and more ARGs were found in pork isolates. There was no difference in the distribution of gyrA point mutations between isolates from chicken and pork. Phylogenetic analysis and sequence type analysis showed that the S. Typhimurium were divided into four clades. The chicken isolates were more genetically polymorphic, and the C4 dominant clades contained both isolates from chicken and pork.

  Conclusion  The antibiotic resistance of S. Typhimurium is common, and the antibiotic resistance of the pathogen from pork is more serious. Antibiotics should be used rationally in animal husbandry. It is necessary to further strengthen the S. Typhimurium surveillance, especially in pork food, for the prevention of resistant pathogens.