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Applied and Environmental Microbiology, September 2007, p. 5789-5796, Vol. 73, No. 18
0099-2240/07/$08.00+0     doi:10.1128/AEM.00533-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Aspartate Biosynthesis Is Essential for the Growth of Streptococcus thermophilus in Milk, and Aspartate Availability Modulates the Level of Urease Activity

Department of Food Science and Microbiology, University of Milan, via Celoria 2, 20133 Milan, Italy,¹ UMR782 Génie et Microbiologie des Procédés Alimentaires, INRA, AgroParisTech, F-78850 Thiverval-Grignon, France,² Central Food Technological Research Institute, 570020 Mysore, India³

Received 8 March 2007/ Accepted 16 July 2007

We investigated the carbon dioxide metabolism of Streptococcus thermophilus, evaluating the phenotype of a phosphoenolpyruvate carboxylase-negative mutant obtained by replacement of a functional ppc gene with a deleted and inactive version, ppc. The growth of the mutant was compared to that of the parent strain in a chemically defined medium and in milk, supplemented or not with L-aspartic acid, the final product of the metabolic pathway governed by phosphoenolpyruvate carboxylase. It was concluded that aspartate present in milk is not sufficient for the growth of S. thermophilus. As a consequence, phosphoenolpyruvate carboxylase activity was considered fundamental for the biosynthesis of L-aspartic acid in S. thermophilus metabolism. This enzymatic activity is therefore essential for growth of S. thermophilus in milk even if S. thermophilus was cultured in association with proteinase-positive Lactobacillus delbrueckii subsp. bulgaricus. It was furthermore observed that the supplementation of milk with aspartate significantly affected the level of urease activity. Further experiments, carried out with a p_ureI-gusA recombinant strain, revealed that expression of the urease operon was sensitive to the aspartate concentration in milk and to the cell availability of glutamate, glutamine, and ammonium ions.

Published ahead of print on 27 July 2007.

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