Question

Which pathway is used by Streptococcus thermophilus for galactose metabolism?

• A. Leloirs pathway
• B. Tagatose 6-p-pathway
• C. Phosphoketolase pathway
• D. Glycolytic pathway

Hint:

Remember the species-specific galactose pathways:
- Streptococcus thermophilus: Leloir pathway.
- Lactococcus lactis: Tagatose 6-phosphate pathway.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Streptococcus thermophilus is a major thermophilic dairy starter culture used in yogurt and cheese making.
During lactose fermentation, the enzyme \(\beta\)-galactosidase cleaves lactose into glucose and galactose.
While glucose is readily metabolized via glycolysis, the utilization of galactose varies among starter strains.

Step 2: Detailed Explanation:
Lactic acid bacteria primarily use one of two pathways for galactose fermentation: the Leloir pathway or the Tagatose 6-phosphate pathway.
- The Tagatose 6-phosphate pathway is typically found in dairy lactococci (such as Lactococcus lactis).
- The Leloir pathway is the primary metabolic route for galactose utilization in Streptococcus thermophilus (for strains that are galactose-fermenting, or \( \text{Gal}^+ \)).
In this pathway, galactose is sequentially converted by galactokinase, galactose-1-phosphate uridylyltransferase, and UDP-glucose 4-epimerase into glucose-1-phosphate, which then enters glycolysis.
Many commercial strains of S. thermophilus are actually galactose-negative (\( \text{Gal}^- \)) and secrete galactose into the medium, but those that do metabolize it utilize the Leloir pathway.
Therefore, option 1 is the correct pathway.

Step 3: Final Answer:
The pathway used by Streptococcus thermophilus for galactose metabolism is the Leloir pathway.