Question

Given below are two statements:
Statement (I) : Lactobacillus delbrueckii ssp. bulgaricus produces D(-) lactic acid
Statement (II) : Streptococcus thermophilus produces L(+) lactic acid.
In light of the above statements, choose the most appropriate answer from the options given below.

• A. Both Statement (I) and Statement (II) are true.
• B. Both Statement (I) and Statement (II) are false.
• C. Statement (I) is true but Statement (II) is false.
• D. Statement (I) is false but Statement (II) is true.

Hint:

Remember the isomer production of the standard yogurt symbiotic pair:
- S. thermophilus produces the physiologically preferred L(+) form.
- L. bulgaricus produces the D(-) form.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Lactic acid exists as two optical isomers: the L(+) isomer and the D(-) isomer.
Mammalian enzymes can easily metabolize the L(+) isomer, whereas the D(-) isomer is metabolized much more slowly.
Different species of lactic acid bacteria possess stereospecific lactate dehydrogenase enzymes, resulting in the production of distinct lactic acid isomers during fermentation.

Step 2: Detailed Explanation:
Let us evaluate the metabolic characteristics of the two classic yogurt starter cultures:
- Statement (I) refers to Lactobacillus delbrueckii ssp. bulgaricus.
This rod-shaped bacterium possesses D-lactate dehydrogenase, which selectively converts pyruvate into the D(-) isomer of lactic acid.
Thus, Statement (I) is true.
- Statement (II) refers to Streptococcus thermophilus.
This coccus-shaped bacterium possesses L-lactate dehydrogenase, which selectively produces the L(+) isomer of lactic acid.
Thus, Statement (II) is true.
Since both statements are biochemically correct, option 1 is the correct selection.

Step 3: Final Answer:
Both Statement (I) and Statement (II) are true.