AUTHOR=Oliveira Ricardo , Pinho Eva , Almeida Gonçalo , Azevedo Nuno F. , Almeida Carina 

TITLE=Prevalence and Diversity of Staphylococcus aureus and Staphylococcal Enterotoxins in Raw Milk From Northern Portugal

JOURNAL=Frontiers in Microbiology

VOLUME=13

YEAR=2022

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2022.846653

DOI=10.3389/fmicb.2022.846653

ISSN=1664-302X

ABSTRACT=<p><italic>Staphylococcus aureus</italic> and staphylococcal enterotoxins are a serious public health concern associated with hospital and community-acquired illnesses. Dairy animals frequently shed <italic>S. aureus</italic> into the milk supply which can lead to food poisoning in humans. This study aims to investigate the prevalence and genetic diversity of <italic>S. aureus</italic> and staphylococcal enterotoxins in raw milk from the main dairy region of mainland Portugal. <italic>S. aureus</italic> was found in 53.0% (95% CI: 40.6–65.4%) of 100 raw cow’s milk samples collected from bulk cooling tanks. The highest contamination level was 3.4 log10 CFU.mL<sup>–1</sup>, and in some samples more than one <italic>S. aureus</italic> strain was identified. Staphylococcal enterotoxins (SEA-SEE) were detected in one sample. Spa typing revealed 62 distinct <italic>S. aureus</italic> isolates, being t529 (17.7%, 95% CI: 8.2–27.3%) and t1403 (16.1%, 95% CI: 7.0–25.3%) the predominant types, commonly associated with livestock infection or carriage. The antimicrobial susceptibility test showed that 35.5% of the <italic>S. aureus</italic> isolates were resistant to at least one antimicrobial agent, with resistance to penicillin being the highest (32.3%, 95% CI: 20.6–43.9%) followed by tetracycline (24.2%, 95% CI: 13.5–34.9%), ciprofloxacin (16.1%, 95% CI: 7.0–25.3%) and chloramphenicol (16.1%, 95% CI: 7.0–25.3%). Moreover, five isolates (8.1%, 95% CI: 1.3–14.8%) were identified as methicillin-resistant <italic>S. aureus</italic> (MRSA, cefoxitin resistant). Regarding virulence/resistance genes, 46,8% (95% CI: 34.4–59.2%) isolates harbored at least one enterotoxin-encoding gene, and the <italic>seg</italic> gene was the most frequently detected (41.9%, 95% CI: 29.7–54.2%) followed by the <italic>sei</italic> (40.3%, 95% CI: 28.1–52.5%), <italic>sec</italic> (6.5%, 95% CI: 0.3–12.6%), <italic>seh</italic> (4.8%, 95% CI: 0.0–10.2%), and <italic>sea</italic> (1.6%, 95% CI: 0.0–4.7%) genes. Five (8.1%, 95% CI: 1.3–14.8%) non-enterotoxigenic isolates carried the <italic>mecA</italic> gene (corresponding to isolates phenotypically classified as MRSA), and 4.8% (95% CI: 0.0–10.2%) enterotoxigenic strains also had the <italic>tsst-1</italic> gene. Our study confirm that raw milk can be a zoonotic source of <italic>S. aureus</italic>, including enterotoxigenic and MRSA strains. Furthermore, the majority of enterotoxigenic isolates were found to contain genes encoding SEs (SEG, SEH and SEI) not routinely screened. This shows the need for a broader SE screening in food safety control, as well as the relevance of risk mitigation measures to control <italic>S. aureus</italic> transmission along the food chain in Portugal.</p>