Occurrence & characterization of Staphylococcus aureus from ready-to-eat (RTE), and cooked food in Singapore: A retrospective analysis

Staphylococcus aureus is one of the major foodborne pathogens that poses significant public health risk, particularly in ready-to-eat (RTE) food prepared under inadequate hygiene conditions. Staphylococcal food poisoning (SFP) is increasingly prevalent worldwide, yet it is not reportable in Singapore, leaving a knowledge gap regarding the clinical incidence and virulence of local strains. This study examined the genetic diversity, antimicrobial resistance (AMR), and virulence determinants of S. aureus in RTE, and cooked food using both conventional testing and whole-genome sequencing (WGS) tools. We retrospectively analyzed 12178 RTE, and cooked food samples collected in Singapore from 2020 to 2023, detecting S. aureus in 0.19 % (23/12178) of samples. Genotype t4171_ST1155 was most common (4/23), followed by t91_ST2990 (3/23). Immunoassays detected the production of classical staphylococcal enterotoxin (SE) A, B, and C types in 43.5 % (10/23) of the isolates. Additionally, genomic analysis identified three more isolates that contained only non-classical SE genes, which were not detectable through immunoassays. AMR occurrence was highest for tetracycline (21.7 %, 5/23), followed by erythromycin (8.7 %, 2/23), clindamycin, gentamicin, and sulfamethoxazole-trimethoprim (4.3 %, 1/23 each). A minority (30.4 %, 7/23) of isolates possessed the complete icaADBC operon, which is associated with the well-studied matrix component involved in biofilm formation. Our findings indicate a low occurrence of S. aureus in RTE, and cooked foods in Singapore, with a minority carrying AMR and biofilm formation features, though about half are enterotoxigenic. This investigation plays a crucial role in comprehending the food safety landscape in Singapore, enabling the development of effective risk mitigation strategies aimed at improving community food safety.