Sugar Utilisation and Conservation of the gal-lac Gene Cluster in Streptococcus thermophilus

Abstract

The adaptation to utilise lactose as primary carbon and energy source is a characteristic for Streptococcus thermophilus. These organisms, however only utilise the glucose moiety of lactose while the galactose moiety is excreted into the growth medium. In this study we evaluated the diversity of sugar utilisation and the conservation of the gal-lac gene cluster in a collection of 18 S. thermophilus strains isolated from a variety of sources. For this purpose analysis was performed on DNA from these isolates and the results were compared with those obtained with a strain from which the complete genome sequence has been determined. The sequence, organisation and flanking regions of the S. thermophilus gal-lac gene cluster were found to be highly conserved among all strains. The vast majority of the S. thermophilus strains were able to utilize only glucose, lactose, and sucrose as carbon sources, some strains could also utilize fructose and two of these were able to grow on galactose. Molecular characterisation of these naturally occurring Gal+ strains revealed up-mutations in the galKTE promoter that were absent in all other strains. These data support the hypothesis that the loss of the ability to ferment galactose can be attributed to the low activity of the galKTE promoter, probably as a consequence of the adaptation to milk in which the lactose levels are in excess.

Molecular and Biochemical Analysis of the Galactose Phenotype of Dairy Streptococcus thermophilus Strains Reveals Four Different Fermentation Profiles

“…In this study, most of the strains that still possessed this A also displayed the Gal-positive phenotype. Recently, van den Bogaard et al (29) also identified two naturally occurring Gal-positive strains that still possessed the original A in the Ϫ10 box. The Gal-negative strains IMDOST05, MB1655, and MB1657 possessed an identical gal promoter sequence as six of the Galpositive strains, indicating that the Gal-positive phenotype is not exclusively determined by the gal promoter sequence.…”

Streptococcus thermophilus strains: Multifunctional lactic acid bacteria

“…The genes involved in galactose and lactose metabolism are located on a single locus (Fig. 3) on the S. thermophilus chromosome, in the order galKTEM lacSZ (Vaillancourt et al, 2002(Vaillancourt et al, , 2004(Vaillancourt et al, , 2008Van den Bogaard, Hols, Kuipers, Kleerebezem, & de Vos, 2004;Vaughan, van den Bogaard, Catzeddu, Kuipers, & de Vos, 2001).…”

Cloning and Overexpression of the als, pflA, and adhB Genes in Streptococcus thermophilus and Their Effects on Metabolite Formation

“…S. thermophilus belongs to the group of the thermophilic lactic acid bacteria and it is used not only by itself but also in combination with other species in dairy fermentations [16]. S. thermophilus can metabolize five different sugars such as lactose, sucrose, glucose, galactose, and fructose [34] and can degrade all amino acids [27]. Therefore, S. thermophilus strains show better growth than L. bulgaricus strains when grown in pure culture.…”