Occurrence and characterization of Salmonella isolates from commercial eggs in Phayao Province, Thailand

Background and Aim: Salmonella contamination in eggs poses a significant public health risk, particularly in alternative egg production systems where contamination and antimicrobial resistance remain underexplored. This study aimed to determine the occurrence of Salmonella contamination in three different egg production systems in Phayao, Thailand, and analyze serovar diversity, antimicrobial resistance, virulence genes, and genetic profiles. Materials and Methods: A total of 750 eggs were sampled from cage, free-range, and organic egg production systems, purchased from supermarkets in Phayao Province. Eggshells and contents were separately analyzed using conventional microbiological methods to isolate Salmonella. Phenotypic identification, serotyping, and antimicrobial susceptibility testing were performed. Genotypic characterization, including virulence and antimicrobial resistance gene detection, was conducted using polymerase chain reaction. Multilocus sequence typing (MLST) was employed to determine genetic diversity. Results: Salmonella contamination was detected in three eggshell samples (0.4%), with one positive sample from each production system. The identified serovars were Salmonella Mbandaka (cage eggs), Salmonella Corvallis (free-range eggs), and Salmonella Cerro (organic eggs). Antimicrobial resistance was observed in only one isolate, S. Mbandaka, which exhibited resistance to sulfamethoxazole/trimethoprim and carried the sul1 and sul2 genes. All Salmonella isolates harbored virulence genes (invA, sopB, and stn). MLST analysis identified three distinct sequence types (ST413, ST1541, and ST1593) corresponding to the detected serovars. Conclusion: This study demonstrates a low occurrence of Salmonella contamination in eggshells across different production systems, with no contamination detected in egg contents. The presence of distinct serovars and genetic types suggests varying contamination sources. Although antimicrobial resistance was minimal, the presence of virulence genes in all isolates highlights the potential risk of infection. Continuous monitoring and improved biosecurity measures in egg production and distribution are recommended to enhance food safety and public health.