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 Leuconostocs, 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 true and (R) is the correct explanation of (A)
• B. Both (A) and (R) are true but (R) is NOT the correct explanation of (A)
• C. (A) is true but (R) is false
• D. (A) is false but (R) is true

Hint:

Remember the genomic location of citrate utilization genes:
- Leuconostoc spp. \(\rightarrow\) Chromosomal-encoded (highly stable).
- Lactococcus lactis biovar. diacetylactis \(\rightarrow\) Plasmid-encoded (unstable, can be lost easily).

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Starter cultures used in fermented dairy products metabolize lactose to produce lactic acid, and some species also metabolize citrate to produce volatile flavor compounds like diacetyl.
This citrate metabolism is regulated by the enzyme citrate permease, which imports citrate into the bacterial cell.

Step 2: Detailed Explanation:
Let us analyze both statements:
- Assertion (A):
Leuconostoc species (such as L. mesenteroides and L. dextranicum) are heterofermentative lactic acid bacteria that metabolize citrate to produce diacetyl, which provides the characteristic buttery aroma in fermented milks.
In contrast, standard strains of Lactococcus lactis subsp. lactis are homofermentative and lack the ability to metabolize citrate.
Only the specific biovar Lactococcus lactis subsp. lactis biovar. diacetylactis is citrate-positive.
Therefore, Assertion (A) is true.
- Reason (R):
In Lactococcus lactis subsp. lactis biovar. diacetylactis, the citrate permease gene (citP) is encoded on a plasmid.
This plasmid is absent in standard Lactococcus lactis subsp. lactis strains, explaining why they cannot utilize citrate.
However, in Leuconostoc species, the genes responsible for citrate transport and utilization are located on the stable bacterial chromosome, not on a plasmid.
Therefore, Reason (R) is incorrect.

Step 3: Final Answer:
Assertion (A) is true, but Reason (R) is false, which corresponds to Option (C).