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.
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 language | English |
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Qualification | Doctor |
Awarding Institution | |
Supervisors/Advisors |
|
Award date | 2010 May 21 |
Publisher | |
ISBN (Print) | 978-91-7422-241-8 |
Publication status | Published - 2010 |
Bibliographical note
Defence detailsDate: 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