Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A): Leuconostocs spp. produce diacetyl from citrate metabolism, but Lactococcus lactis spp. lactis do not do the same.
Reason (R): The plasmid coding for citrate utilization genes are present in Leuconostoc, but not in Lactococcus lactis spp. lactis.
In light of the above statements, choose the most appropriate answer from the options given below:

• A. Both (A) and (R) are correct and (R) is the correct explanation of (A).
• B. Both (A) and (R) are correct but (R) is NOT the correct explanation of (A).
• C. (A) is correct but (R) is not correct.
• D. (A) is not correct but (R) is correct.

Hint:

Remember: "Leuconostoc" = Flavor (Diacetyl). "Lactococcus lactis" = Acid. Diacetyl production is a plasmid-linked trait in dairy starter cultures.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Starter cultures in the dairy industry, specifically those used for butter and certain cheeses, are chosen for their ability to produce flavor compounds.
Diacetyl is the primary compound responsible for the characteristic "buttery" aroma. It is produced through the fermentation of citrate present in milk.

Step 2: Detailed Explanation:
1.Analysis of Assertion (A): Leuconostoc species (like L. mesenteroides subsp. cremoris) are heterofermentative lactic acid bacteria known as "aroma bacteria" because they actively ferment citrate to produce diacetyl and $CO_{2}$. Lactococcus lactis subsp. lactis, however, is a homofermentative acid producer that primarily ferments lactose to lactic acid and does not utilize citrate for diacetyl production. Thus, the assertion is correct.
2.Analysis of Reason (R): The ability to transport and metabolize citrate in lactic acid bacteria is often mediated by genes located on extrachromosomal DNA known as plasmids. In Leuconostoc, the citrate permease and associated genes are present (allowing diacetyl production). In standard Lactococcus lactis subsp. lactis, these specific plasmids are absent. (Note: Only L. lactis subsp. lactis biovar. diacetylactis possesses the plasmid for citrate utilization). Thus, the reason is correct.
3.Relationship: The presence or absence of the specific genetic code (the plasmid) is exactly why one species can produce diacetyl while the other cannot. Therefore, (R) is the correct explanation for (A).

Step 3: Final Answer:
Both (A) and (R) are correct and (R) is the correct explanation of (A).