Whole-Genome Sequencing-Based Characterisation of Salmonella Saintpaul Isolates from Human and Food Sources in Singapore: Sequence Types, Antimicrobial Resistance and Plasmid Profiles.

Salmonella Saintpaul is a foodborne pathogen of public health relevance. This study characterised 79 S. Saintpaul isolates from human clinical and food sources collected in Singapore from 2015 to 2017. Isolates belonged to ST3602 (91.1%, n = 72), ST27 (6.33%, n = 5), ST49 (1.27%, n = 1) or ST50 (1.27%, n = 1). ST3602 predominated in both human (92.5%) and food (89.7%) isolates. Human and food isolates showed the highest resistance to penicillins and beta-lactam/beta-lactamase inhibitor combinations. The analysis identified beta-lactam resistance genes (blaCTX-M-1, blaTEM-1D), aminoglycoside resistance genes (aph(4)-Ia, aac(3)-IVa, aadA), and fluoroquinolone resistance genes (qnrS) as the most prevalent. Notably, 31.6% of isolates identified as extended-spectrum beta-lactamase (ESBL)-producing and all ESBL-positive isolates exhibited multidrug resistance. Plasmids were predicted in 75.9% (n = 60) of the isolates, with IncI1-I(Alpha) being the most common incompatibility group and showing a strong association with blaCTX-M-1 (Fisher's exact test, p < 0.001; Φ = 0.913). Single-nucleotide polymorphism (SNP) analysis revealed close genetic relatedness between chicken and human isolates at relatively low SNP distances (0-16 SNPs), suggesting potential shared transmission pathways. This study provides epidemiological insights and antimicrobial resistance landscape of S. Saintpaul in Singapore, informing strategies for food safety and public health surveillance.