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Galactose Metabolism and Capsule Formation in a Recombinant Strain of Streptococcus thermophilus with a Galactose-Fermenting Phenotype

G. Robitaille^*,1, S. Moineau^,,, D. St-Gelais^*, C. Vadeboncoeur^, and M. Britten^*

^* Food Research and Development Centre (FRDC), Agriculture and Agri-Food Canada, St-Hyacinthe, Quebec, Canada, J2S 8E3
Département de biochimie et de microbiologie, Faculté des sciences et de génie,
Groupe de recherche en écologie buccale (GREB), Faculté de médecine dentaire, and
Félix d’Hérelle Reference Centre for Bacterial Viruses, Université Laval, Quebec City, Quebec, Canada, G1K 7P4

¹ Corresponding author: robitaillegi{at}agr.gc.ca

The capsule-producing, galactose-negative Streptococcus thermophilus MR-1C strain was first transformed with a low-copy plasmid containing a functional galK gene from Streptococcus salivarius to generate a recombinant galactose-fermenting Strep. thermophilus strain named MR-AAC. Then, we compared the functional properties of Strep. thermophilus MR-AAC with those of the parent MR-1C strain when used as starter for fermented products and cheese. In lactose-supplemented laboratory medium, MR-AAC metabolized galactose, but only when the amount of lactose was less than 0.1% (wt/vol). After 7 h of fermentation, the medium was almost depleted of galactose. The parent strain, MR-1C, showed the same pattern, except that the concentration of galactose decreased by only 25% during the same period. It was found that, during milk fermentation and Mozzarella cheese production, the galactose-fermenting phenotype was not expressed by MR-AAC and this strain expelled galactose into the medium at a level similar to the parent MR-1C strain. In milk and in lactose-supplemented medium, capsular exopolysaccharide production occurred mainly during the late exponential phase and the stationary growth phase with similar kinetics between MR-1C and MR-AAC.

Key Words: capsular exopolysaccharide • galactose metabolism • cheese • Streptococcus thermophilus

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				K. Vaillancourt, N. Bedard, C. Bart, M. Tessier, G. Robitaille, N. Turgeon, M. Frenette, S. Moineau, and C. Vadeboncoeur Role of galK and galM in Galactose Metabolism by Streptococcus thermophilus Appl. Envir. Microbiol., February 15, 2008; 74(4): 1264 - 1267. [Abstract] [Full Text] [PDF]