Characterisation of Classical Enterotoxins, Virulence Activity, and Antibiotic Susceptibility of Staphylococcus Aureus Isolated from Thai Fermented Pork Sausages, Clinical Samples, and Healthy Carriers in Northeastern Thailand

Abstract

Introduction: Contamination by Staphylococcus aureus of food produced from animal sources may have diverse and multifactorial causes that depend on geographical distribution. The goal of this study was to isolate and characterise S. aureus strains from contaminated fermented pork sausage, which is a local food of northeastern Thailand.

Material and methods: S. aureus strains were isolated from local pork sausage, and the presence of classical enterotoxins was determined by PCR and reversed passive latex agglutination. These results were compared with strains derived from hospitalised patients and healthy carriers. Additionally, production of extracellular enzymes and haemolysin, biofilm formation, and antibiotic susceptibility were assessed.

Results: S. aureus was identified in 36 sausage isolates (60%). The strains positive for staphylococcal enterotoxin A were more frequently found in isolates from sausage and healthy carriers than in those from patients. All tested S. aureus strains were positive for DNase, lipase, proteinase, haemolysin, and biofilm formation; notably, strains isolated from food and healthy carriers displayed similar values. Most isolates were resistant to penicillin and ampicillin, while none were to methicillin.

Conclusions: Thai fermented pork sausages are associated with a high risk of staphylococcal food poisoning, which may be linked to contamination caused by carriers. Dissemination of knowledge regarding best practices in sanitation and hygiene is important in local communities.

Keywords: Staphylococcus aureus enterotoxins; Thailand; food; pork.

Fig. 1

PCR amplification of a S. aureus specific gene (nuc), classical SE genes (sea–see), and the tsst-1 gene. Lane M – 100 bp DNA marker; Lanes 1–7 – nuc, sea, seb, sec, sed, see, and tsst-1, respectively

Fig. 2

Production of extracellular enzymes (DNase, lipase, protease) and haemolysin by S. aureus strains isolated from three different sources. Data are presented as the mean diameter of clear zones surrounding colonies (mm), determined from triplicate independent experiments. * P < 0.05 is considered to be statistically significant in between-group comparisons

Fig. 3

Biofilm formation was evaluated by crystal violet staining in triplicate independent experiments. Absorbance at 570 nm was measured with a microplate reader. Isolates with OD₅₇₀ values ≥0.76 were considered to be strong biofilm formers. * P < 0.05 was considered to be significantly different in between-group comparisons