Foodborne Virulence: Staphylococcus aureus Enterotoxins A and D

Nina Wallin

    Research output: ThesisDoctoral Thesis (compilation)

    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.
    Original languageEnglish
    QualificationDoctor
    Awarding Institution
    Supervisors/Advisors
    • Rådström, Peter, Supervisor
    Award date2010 May 21
    Publisher
    ISBN (Print)978-91-7422-241-8
    Publication statusPublished - 2010

    Bibliographical note

    Defence details

    Date: 2010-05-21
    Time: 10:15
    Place: Lecture hall B, at the Center of Chemistry and Chemical Engineering, Getingevägen 60, Lund University Faculty of Engineering

    External reviewer(s)

    Name: Jespersen, Lene
    Title: Professor
    Affiliation: Faculty of Life Sciences, University of Copenhagen, Denmark

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    Subject classification (UKÄ)

    • Industrial Biotechnology

    Free keywords

    • environmental factors
    • phage-encoded virulence
    • in vivo
    • enterotoxin D
    • enterotoxin A
    • gene expression
    • Staphylococcus aureus
    • virulence

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