Abstract
The amino acid metabolism of lactic acid bacteria used as starters in industrial
fermentations has profound effects on the quality of the fermented foods. The work described
in this PhD thesis was initiated to use genomics technologies and a comparative approach to
link the gene content of some well-known lactic acid bacteria to flavor formation and to
increase our general knowledge in the area of amino acid metabolism. The three well-known
lactic acid bacteria that were used in these studies were Streptococcus thermophilus,
Lactococcus lactis and Lactobacillus plantarum. The complete genomes of all these model
bacteria have been sequenced and annotated in detail.
Comparative experimental and in silico studies of Streptococcus thermophilus with the
other two lactic acid bacteria, revealed the low degree of amino acid auxotrophies of this
species; it only needs two amino acids for (minimal) growth and this strain is able to produce
a varied amount of flavors. Lactococcus lactis and Lactobacillus plantarum require more
amino acids and produce fewer flavors than S. thermophilus. Furthermore, S. thermophilus
has a simple primary metabolism; homolactic growth is the only possible route under
anaerobic conditions and, remarkably, it does not have a complete pentose phosphate pathway
in contrast to the other two studied bacteria. This latter property has important consequences
for the redox metabolism of S. thermophilus and particularly its ability to produce NADPH.
A genome-scale metabolic model was developed and predicted that amino acid metabolism,
and especially glutamate degradation, and citrate metabolism are the most obvious
alternatives for NADPH generation. Several of these predictions were confirmed by
constructing a glutamate dehydrogenase mutant of S. thermophilus. This mutant revealed theimportance of the citrate pathway (and other amino acid degradation pathways) in NADPH generation.
A comparative and functional genomics study of the three lactic acid bacteria showed
that amino acid depletion not only affects amino acid metabolism, but also flavor formation
and overall growth. The comparative genomics approach presented in this thesis can be used
to understand the amino acid metabolism of different lactic acid bacteria and their potential to
produce flavors under different conditions. Finally, it can be applied for optimization of
industrial fermentations
Original language | English |
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Qualification | Doctor of Philosophy |
Awarding Institution |
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Supervisors/Advisors |
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Award date | 16 Oct 2009 |
Place of Publication | [S.l. |
Print ISBNs | 9789085854616 |
DOIs | |
Publication status | Published - 16 Oct 2009 |
Keywords
- amino acid metabolism
- lactic acid bacteria
- streptococcus thermophilus
- comparative genomics
- functional genomics