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Press Info item.
16/06/2006
The sequence of the lactobacillus genome in yogurt unveiled
A research team from INRA at Jouy-en-Josas, in collaboration with Genoscope, has sequenced and analysed the genome of one of the two bacteria in yogurt: Lactobacillus bulgaricus. The mysteries of yogurt are little by little being unveiled, since the genome of the other bacteria specific to yogurt, Streptococcus thermophilus, has also been sequenced – a study managed and conducted in part by the same laboratory. This type of study could have an important impact on the control of industrial fermentation processes.
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Yogurt, a product of milk fermentation
Yogurt, very much present in our diet today, has been known and appreciated for a long time: the first references to this product date back to 3200 BC. found on clay tablets discovered in Irak. Yogurt is the product of milk fermentation by two lactic acid bacteria: Lactobacillus bulgaricus and Streptococcus thermophilus.
Lactic acid and the other molecules that are formed during fermentation make yogurt a food product that is both acidic and creamy, appreciated for its taste and nutritional qualities, notably for its calcium content. From a microbiological viewpoint, due to its acidity, yogurt may be conserved for much longer periods of time than milk. Yogurt is thus a very convenient food as compared to milk, which is very fragile.
At the beginning of last century, Metchnikoff put forward the hypothesis that the live bacteria of yogurt have a beneficial effect on health. At that time, yogurt was prescribed for the treatment of certain diseases. This same concept has currently been adopted under the term « probiotics ». Currently, lactobacilli close to L. bulgaricus are notably used in association with bacteria from yogurt in probiotic milk products.
© INRA/M. Rousseau - PCD0668-IMG0017.PCD
Yogurt results from the fermentation of milk by two bacteria: Lactobacillus bulgaricus (rod cells) and Streptococcus thermophilus (coccus cells).
L. bulgaricus: the deciphering of its DNA
The genome of the bacteria L. bulgaricus, is made up of approximately 1.9 million bases. It presents several characteristics of evolution and a rapid adaptation to the environment of milk. The most evident is the inactivation of more than 250 genes out of 1800. The inactivation of many genes has also been observed in S. thermophilus. The lost functions attest to the specialisation of lactobacillus, which seems to have passed from a plant environment to a milk environment. Several metabolic pathways of sugars of plant origin have been inactivated in order to favour the specific use of lactose, the sugar that is present in milk. In the same way, almost the whole capacity of biosynthesis of the amino acids has been lost in favour of the use of milk proteins. Finally, the high amounts of ribosomic operons, genetic elements essential to the life of the bacteria, suggest that the genome size of these bacteria has decreased significantly.
This evolution contributes to the differentiation of L. bulgaricus from other similar lactobacilli: a comparison of genetic elements revealed around 2/3 of genes common to the lactobacillus family and 1/3 of the genes specific to L. bulgaricus. Some of these specific genes could play a role in the adaptation of milk bacteria.
Besides its specialisation towards a milk environment, L. bulgaricus also seems adapted to its indissociable yogurt partner, S. thermophilus. For the synthesis of specific molecules, the biosynthesis pathways may be shared by the two microorganisms: the inactivated steps in L. bulgaricus may be taken care of by S. thermophilus and vice-versa. For example, for the synthesis of folate (vitamin B), S. thermophilus could intervene so that the molecule can be synthesised by L. bulgaricus.
The knowledge of the genome sequences of the two bacteria of yogurt opens the way to studies aimed at a better understanding of the collaboration of both, as well as the analysis of the dynamics of cellular processes during fermentation. Eventually, this type of study could have an important impact on the control of industrial fermentation processes.
© INRA/M. Van de Guchte
Atlas of the L. bulgaricus genome
The seven circles (from the outside to the inside) show:
- Circle 1, the inactivated genes (pseudogenes) on the positive strand (red) and the negative strand (blue).
- Circle 2, the IS elements.
- Circle 3, the coding sequences (excluding pseudogenes and transposases) on the positive strand (red) or the negative strand (blue).
- Circle 4, the genes coding for ribosomal RNA (rRNA, red) and transfer RNA (tRNA, green).
- Circle 5, [(G-C)/width of the window (2000)], less than -0.1 (cyan) to more than +0.1 (red).
- Circle 6, [(A+T)/width of the window (500)], less than 0.3 (cyan) to more than 0.7 (red).
- Circle 7, position on the genome.
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Sources :
The complete genome sequence of Lactobacillus bulgaricus reveals extensive and ongoing reductive evolution. M. van de Guchte et al. (2006) Proc. Natl. Acad. Sci. 13, 9274-9279.
Complete genome sequence and comparative analysis of the dairy bacterium Streptococcus thermophilus. A. Bolotin et al. (2004) Nat. Biotechnol. 22, 1554-1558.
Scientific contacts:
Maarten VAN DE GUCHTE
tel.: 33 1 34 65 25 28 or 33 1 34 65 25 11
Maarten.VanDeGuchte@jouy.inra.fr
or Emmanuelle MAGUIN
tel.: 33 1 34 65 25 18 or 33 1 34 65 25 11
Emmanuelle.Maguin@jouy.inra.fr
Microbial Genetics Research Unit, Microbiology and the Food Chain Department, INRA Research Centre of Jouy-en-Josas.
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Written by :
INRA press service, phone: +33 (0)1 42 75 91 69
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