Foodborne Virulence: Staphylococcus aureus Enterotoxins A and D
Abstract: The development of new, minimally processed food products challenges traditional concepts of food safety. How pathogenic bacteria behave in these new matrices is not known. To fill this knowledge gap and enable the production of food that is safe for the consumer, more information on virulence expression of pathogens in food matrices is required. In this thesis, the impact of environmental factors on foodborne intoxication due to Staphylococcus aureus enterotoxin A (SEA) and D (SED) expression and production is described. The study of Campylobacter jejuni infection of the chicken GI tract was used to compare the different mechanisms of virulence of the two foodborne pathogens. Studies on S. aureus demonstrated that under certain conditions, e.g. low pH and lack of nutrients, the SEA production per cell was increased. In the first part of these studies, when S. aureus was grown in the presence of acetic acid, under controlled laboratory conditions; the sea expression pattern was similar at all pH values studied, with the expression peaking in the transition from exponential to stationary growth phase, then falling in the stationary growth phase. The sea expression was upregulated over a range of acetic acid concentrations, and the increased expression could be linked to the activity of the seacarrying prophage, showing that the prophage participated in the regulation of virulence expression. A nucleotide sequence analysis of the virulence region of six S. aureus strains carrying the sea gene showed specific sea phage groups and two versions of the sea gene that may explain the observed interstrain variance in SEA expression and production levels. S. aureus growth and SEA and SED expression and production were then studied in vivo in four pork meat products. The intrinsic nature of the meat products greatly affected the growth and expression patterns of the organism. The number of S. aureus increased rapidly on the boiled and smoked ham products. However, on the Serrano ham the number of S. aureus cells did not increase until after seven days of incubation and the microorganism did not survive on the salami investigated. In boiled and smoked hams, active sea and sed expression was detected throughout the one-week experiment, while the enterotoxin expression in pure culture peaked after only a few hours of cultivation and then decreased to low levels during the rest of the study. The SEA and SED levels on boiled ham decreased unexpectedly after five days of incubation, maybe due to extracellular proteolytic activity of S. aureus or lactic acid bacteria. Finally, the in vivo virulence of a pathogen causing foodborne infection was studied by monitoring the dynamics of C. jejuni colonization of the alimentary tract in chicken. Seven C. jejuni strains simultaneously colonized two types of chicken, with different gastrointestinal floras. The colonization patterns were the same in both types of chicken, despite the different microbiota, showing that the background flora had no major effect on the colonization order. Instead, it was suggested that factors concerning the host, probably the host’s immune response, affected the C. jejuni population during infection. A greater understanding of how bacterial growth and virulence expression are related and regulated by environmental factors and food preservatives will provide safer food products and give rise to new approaches to disease prevention and control in the future through the improvement of quantitative risk assessments. The virulence of foodborne pathogens can be an important complement to the viable counts traditionally used in food safety assessments.
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