Chapter 18. Staphylococcus aureus

Staphylococcus aureus is a Gram-positive coccus, which tends to be arranged in irregular clusters or grape-like clusters when viewed through a microscope and has large, round, golden-yellow colonies, often with hemolysis, when grown on blood agar plates. Staphylococcal food poisoning (SFP) relies on one single type of virulence factor: the SEs. Risk assessment in foodstuffs relies on classical microbial detection and quantification of coagulase positive staphylococci on a selective Baird-Parker medium, whose composition is standardized. Use of DNA-based assays may circumvent some of the problems associated with conventional microbiological procedures. Perhaps the greatest single advantage of DNAbased diagnostic assays is that these methods focus on the unique nucleic acid composition of the bacterial genome rather than on phenotypic expression of products that nucleic acids encode. Therefore, DNA-based identification assays are subject to less variability compared with diagnostic methods based on phenotypic characterization, allowing reliable detection and quantification down to one single nucleic acid target per PCR sample. Moreover, not only the presence of the pathogen but also of the genes encoding for SEs production is important to evaluate as enterotoxins nonproducing strains may also occur.