Antimicrobial Resistance and Molecular Characterization of Salmonella enterica Serovar Enteritidis from Retail Chicken Products in Shanghai, China.

Salmonella enterica serovar Enteritidis is the leading global cause of salmonellosis. A total of 146 Salmonella Enteritidis isolates obtained from retail chicken products in Shanghai, China were characterized for their antimicrobial susceptibilities, virulence and antibiotic resistance gene profiles, and molecular subtypes using pulsed-field gel electrophoresis (PFGE). Approximately 42% (61/146) of the isolates were susceptible to all 13 antimicrobials tested. More than half of the isolates (50.70%) were resistant to ampicillin, 49.32% to sulfisoxazole, 17.12% to tetracycline, and 15.75% to doxycycline. Thirty (20.55%) isolates were resistant to three or more antimicrobials. The avrA, mgtC, and sopE virulence genes were identified in all isolates, while 97.2% and 92.4% were positive for bcfC and spvC genes, respectively. Genes associated with resistance to streptomycin (aadA), β-lactams (blaTEM, blaCMY, blaSHV, and blaCTX), tetracycline (tetA and tetB), and sulfonamides (sulI, sulII, and sulIII) were detected among corresponding resistant isolates. A total of 41 PFGE patterns were identified from 77 antimicrobial resistance (AMR) isolates and were primarily grouped into seven clusters (A-G), each with 90% similarity. The majority of Salmonella Enteritidis isolates (63.63%, 49/77) shared the same PFGE cluster, indicating potential cross contamination during processing and cutting or working during retailing and marketing. A significantly (p < 0.05) lower percentage (<25%) of isolates belonging to clusters D and E were resistant to sulfisoxazole compared with those belonging to clusters A, B, C, F, and G (>80%), indicating that sulfisoxazole resistance might be associated with genetic content (PFGE profiles) of Salmonella Enteritidis. This study provides important and updated information about the baseline antimicrobial-resistant data for food safety risk assessment of Salmonella Enteritidis from retailed chicken in Shanghai, which is the first step for the development and implementation of China's AMR National Action Plan, and can be helpful for future surveillance activities to ensure the safety of the chicken supply.