Staphylococcus aureus (S. aureus) is a notorious opportunistic foodborne patho¬gen and also a common cause of bovine mastitis. It is known to produce many different virulence factors, including various staphylococcal enterotoxins and toxic shock syndrome toxin-1 (TSST-1). The aim of this work was to investigate the regulation, expression, formation and role of S. aureus toxins in relation to food safety and animal health.
The link between the expression of the sea gene and the formation of enterotoxin A (SEA) and the life cycle of the sea-bearing Siphoviridae bacteriophages was investigated. Variations in SEA concentration between different strains of S. aureus were found to depend on the induction capacity of the phage, rather than sequence differences in the sea region. The SOS system, which responds to DNA damage in the cell, was also shown to be linked to the phage life cycle through the RecA protein.
A novel method of monitoring SEA formation in real-time using flow cytometry was developed by fusing a green fluorescent protein (GFP) reporter to SEA. The mean value of the fluorescence intensity of the cells was shown to correspond to the SEA concentration determined by ELISA. This method was used to monitor SEA formation in the presence of different concentrations of NaCl, and has the potential to be further developed and optimized for rapid, large-scale screening of the effect of different environmental factors on toxin formation.
The effect of sodium nitrite (NaNO2) on the expression of the sed gene and the formation of enterotoxin D (SED) was investigated in three different wild type S. aureus strains and their isogenic Agr, SarA and SigB regulatory mutants. The investigation revealed that SED is not as strongly dependent on the Agr system as previously reported. SED expression and formation were also measured in an Agr deletion mutant grown on boiled ham. The results showed significant differences in SED formation compared to growth in an optimal liquid broth, emphasizing the import¬ance of performing experiments on real food environment.
The function of enterotoxin C (SEC) and TSST-1 in bovine mastitis was studied using an experimental model with bovine mammary epithelial cells. S. aureus SEC and TSST-1 deletion mutants showed significantly reduced infection capability compared to the isogenic wild type strains. This indicates that toxins may play a role during the establishment of infection, and should be further investigated.
A deeper understanding of the regulation, formation and function of S. aureus toxins will help improve food safety and advance the development of prevention strategies and treatments for bovine mastitis in the future.