Question

Galactose fermentation is used to distinguish

• A. Lactobacillus helveticus and L. delbrueckii ssp. bulgaricus
• B. Lactobacillus acidophilus and L. delbrueckii ssp. bulgaricus
• C. Lactobacillus helveticus and L. casei
• D. Lactobacillus casei and Lactobacillus acidophilus

Hint:

Thermophilic Starter Differentiation:
Lactobacillus helveticus = GALACTOSE FERMENTING ($ ext{Gal}^+$) $\rightarrow$ Prevents cheese browning.
L. delbrueckii subsp. bulgaricus = GALACTOSE NON-FERMENTING ($ ext{Gal}^-$).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Carbohydrate metabolic profiling in thermophilic lactobacilli: Lactobacillus helveticus ferments both glucose and galactose (Gal+), whereas Lactobacillus delbrueckii subsp. bulgaricus ferments glucose but excretes galactose into the medium (Gal-).
Key Formula or Approach:
\[ \text{Galactose Phenotype: } \textit{Lactobacillus helveticus } (\mathbf{Gal^+}) \quad \longleftrightarrow \quad \textit{L. delbrueckii} \text{ ssp. } \textit{bulgaricus } (\mathbf{Gal^-}) \]

Step 2: Detailed Explanation:
In the physiological taxonomy and sugar fermentation profiling of thermophilic dairy lactobacilli:
- Both Lactobacillus helveticus and Lactobacillus delbrueckii subsp. bulgaricus are major thermophilic starter organisms utilized in Swiss cheese, Mozzarella, and yoghurt:
1. {Lactobacillus helveticus (A): Possesses the Leloir pathway (galactokinase and UDP-galactose-4-epimerase); it readily metabolizes and ferments Galactose ($ ext{Gal}^+$ positive), clearing galactose from curd and preventing Maillard browning defect in baked Mozzarella cheese.
2. Lactobacillus delbrueckii subsp. bulgaricus: Lacks enzymes for galactose catabolism; it takes up lactose, hydrolyzes it with $\beta$-galactosidase, ferments only the glucose moiety, and excretes galactose into the extracellular medium ($ ext{Gal}^-$ negative, cannot ferment galactose).
- Hence, Galactose fermentation is the primary diagnostic phenotypic test to distinguish between these two species.

Step 3: Final Answer:
Thus, galactose fermentation is used to distinguish L. helveticus and L. delbrueckii ssp. bulgaricus, matching option (A).