Dissertation committee:
Ν. Γαλανοπούλου, Καθηγήτρια ΕΚΠΑ Π. Μαρκάκη, Αναπληρώτρια Καθηγήτρια ΕΚΠΑ (Επιβλέπουσα) Χ. Προεστός, Επίκουρος Καθηγητής ΕΚΠΑ
Summary:
Listeria monocytogenes is preσεσντ in a variety of foods and ready-to-eat
products and it may generate a serious public health problem. The ability of
L.monocytogenes to be adaptedat low temperature, low pH, osmotic stress, to
withstand cleaning and disinfection agents and to perform cross-protection,
combined with its ability to form biofilms, allow it to prevail over commonly
used techniques of food safety. Among the strategies undertaken by
L.monocytogenes to adapt to environmental changes, modifications of membrane
lipids are essential for keeping enough fluidity compatible with integrity and
functionality of the membrane.
The objective of the present study was to determine the changes that occur in
lipid composition of the membrane of L.monocytogenes during its adaptation to
the manufacture and storage of a laboratory prepared soft cheese (feta type).
This will result in a deeper understanding of bacterium’s mechanisms to
maintain the membrane fluidity in adverse conditions.
The results show that L.monocytogenes being present in milk (5 log cfu/ml) may
survive the manufacture and storage of cheese. Although the surviving level of
stressed cells (3 log cfu/g) differ from the level of cells grown in optimal
conditions (7 log cfu/g), it is still above the regulatory levels throughout
the experimental storage. Moreover, the history of inoculated cells has an
effect on their behavior during storage of the cheese.The bacterium seems to
follow different ways as a function of time and of the inocula history in order
to maintain optimum membrane fluidity, and hence the possible operation of the
membrane to the conditions of this experiment (preparation and storage of
cheese). So, cells derived from inoculated cells grown in optimal conditions
used the increase of straight chain fatty acids (SCFA) at the expence of
branched chain fatty acids (BCFA), without altering the ratio of branched C15:0
to branched C17:0 fatty acids, and the ratio of iso to anteiso fatty acids.
These changes were maintained during all experimental period. the cells
derived from inoculated cells grown in stressed conditions follow a different
way. The balance between fluid and solid phase membrane was achieved not only
by changes in SCFA, but also by changes in the ratios of br-C15: 0 / br-C17: 0
and iso to anteiso fatty acids.
Keywords:
Listeria monocytogenes, Lipids, Fatty acids, Survival, Feta cheese