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Applied and Environmental Microbiology, August 2004, p. 4596-4603, Vol. 70, No. 8
0099-2240/04/$08.00+0     DOI: 10.1128/AEM.70.8.4596-4603.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

Characterization of a Galactokinase-Positive Recombinant Strain of Streptococcus thermophilus

Katy Vaillancourt,1 Jean-Dominique LeMay,1 Maryse Lamoureux,2 Michel Frenette,1 Sylvain Moineau,1 and Christian Vadeboncoeur1*

Groupe de Recherche en Écologie Buccale (GREB), Département de Biochimie et de Microbiologie, Faculté des Sciences et de Génie, and Faculté de Médecine Dentaire, Université Laval, Quebec City, Quebec, Canada G1K 7P4,1 Agropur, Granby, Quebec, Canada J2G 7G22

Received 3 February 2004/ Accepted 21 April 2004

The lactic acid bacterium Streptococcus thermophilus is widely used by the dairy industry for its ability to transform lactose, the primary sugar found in milk, into lactic acid. Unlike the phylogenetically related species Streptococcus salivarius, S. thermophilus is unable to metabolize and grow on galactose and thus releases substantial amounts of this hexose into the external medium during growth on lactose. This metabolic property may result from the inability of S. thermophilus to synthesize galactokinase, an enzyme of the Leloir pathway that phosphorylates intracellular galactose to generate galactose-1-phosphate. In this work, we report the complementation of Gal– strain S. thermophilus SMQ-301 with S. salivarius galK, the gene that codes for galactokinase, and the characterization of recombinant strain SMQ-301K01. The recombinant strain, which was obtained by transformation of strain SMQ-301 with pTRKL2TK, a plasmid bearing S. salivarius galK, grew on galactose with a generation time of 55 min, which was almost double the generation time on lactose. Data confirmed that (i) the ability of SMQ-301K01 to grow on galactose resulted from the expression of S. salivarius galK and (ii) transcription of the plasmid-borne galK gene did not require GalR, a transcriptional regulator of the gal and lac operons, and did not interfere with the transcription of these operons. Unexpectedly, recombinant strain SMQ-301K01 still expelled galactose during growth on lactose, but only when the amount of the disaccharide in the medium exceeded 0.05%. Thus, unlike S. salivarius, the ability to metabolize galactose was not sufficient for S. thermophilus to simultaneously metabolize the glucose and galactose moieties of lactose. Nevertheless, during growth in milk and under time-temperature conditions that simulated those used to produce mozzarella cheese, the recombinant Gal+ strain grew and produced acid more rapidly than the Gal– wild-type strain.

* Corresponding author. Mailing address: Groupe de Recherche en Écologie Buccale (GREB), Département de Biochimie et de Microbiologie, Faculté des Sciences et de Génie, and Faculté de Médecine Dentaire, Université Laval, Quebec City, Quebec, Canada G1K 7P4. Phone: (418) 656-2319. Fax: (418) 656-2861. E-mail: Christian.Vadeboncoeur{at}bcm.ulaval.ca.

Applied and Environmental Microbiology, August 2004, p. 4596-4603, Vol. 70, No. 8
0099-2240/04/$08.00+0     DOI: 10.1128/AEM.70.8.4596-4603.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.



This article has been cited by other articles:

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