Comparison of amino acid metabolism in S. thermophilus with other
lactic acid bacteria
Marieke Pastink, Bas Teusink, Sanne Visser, Willem M. de Vos, Jeroen Hugenholtz
Kluyver Centre for Genomics of Industrial Fermentation, Top Institute Food&Nutrition, NIZO
food research, PO Box 20, 6710 BA Ede. The Netherlands
Tel +31 (0)318-659533
Email: marieke.pastink@nizo.nl
Lactic acid bacteria are important for the dairy industry as (mixed) starter cultures for the
production of fermented foods, such as yoghurt and cheese. Typically, mixed starters
consisting of Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus are
used to ferment milk yielding the final yoghurt product. S. thermophilus is also used for the
production of specific cheeses using elevated temperatures (3).
In this project, we compare amino acid metabolism in S. thermophilus strain LMG18311, with
other well-studied lactic acid bacteria. The genome of this strain has been sequenced and
extensively studied by the group of P. Hols (1, 2). It has a high level of pseudogenes (around
10%); most of these pseudogenes are related to carbohydrate metabolism and to virulence.
Some of these pseudogenes are associated with amino acid biosynthesis (1).
We first established amino acid-dependency for growth of S. thermophilus. In growth
experiments using chemically defined medium (CDM), we determined that S. thermophilus
LMG18311 needs only histidine and one of the sulphur-containing amino acids for (minimal)
growth. GC-MS analyses of S. thermophilus cultures grown on CDM showed that this lactic
acid bacterium is able to produce a broad variety of amino acid-derived flavours, also
reflecting a relatively complete set of amino acid biosynthetic and amino acid-converting
pathways.
A genome-scale metabolic model for S. thermophilus has been developed. This model is
based on the annotated genome in which genes encoding for metabolic enzymes have been
identified and associated to the corresponding reactions. Moreover, the biomass composition
of this strain was determined. Organic compounds and amino acids in fermentation samples
were measured by HPLC and fluxes were calculated (4). The model is now at a stage where
in silico growth can be simulated under different conditions. The model strongly suggests that
homofermentative lactic acid production is the only primary metabolism operating in S.
thermophilus and this is confirmed by fermentation results under various cultivation
conditions. Other interesting observations instigated by experimental data and the genomescale model are the unique production pathway for acetaldehyde (the yoghurt flavour) and
the absence of the pentose phosphate pathway.
References:
1. Bolotin et al, Nature biotechnology 22, 1554-1558. 2004
2. Hols et al, FEMS microbiology reviews 29, 435-463. 2005
3. Pastink et al, International Dairy Journal. Article in press. 2007
4. Teusink et al, Journal of Biological Chemistry 281, 40041-40048. 2006