Q.77 Determine the correctness or otherwise of the following Assertion [a] and the
Reason [r].
Assertion [a]: Endospore can survive heat that would rapidly kill vegetative cells of
the same species.
Reason [r]: In endospore, the protoplasm is reduced to minimum volume as a result
of the accumulation of calcium–dipicolinic acid complexes and small acid–soluble
spore proteins, which forms a cytoplasmic gel and a thick cortex.
(A) Both [a] and [r] are true and [r] is the correct reason for [a]
(B) Both [a] and [r] are true and [r] is not the correct reason for [a]
(C) Both [a] and [r] are false
(D) [a] is true but [r] is false

Endospores exhibit remarkable heat resistance compared to vegetative cells due to specific structural modifications during sporulation. The assertion and reason provided in the question both hold true, but the reason does not fully explain the heat survival mechanism.

Assertion Analysis

The assertion states that endospores survive heat that kills vegetative cells of the same species. This is correct, as endospores of bacteria like Bacillus subtilis withstand temperatures of 100°C or higher for minutes, while vegetative cells die rapidly due to protein denaturation and membrane damage.​

Reason Evaluation

The reason claims protoplasm reduces to minimum volume from calcium-dipicolinic acid (Ca-DPA) complexes and small acid-soluble spore proteins (SASPs), forming a cytoplasmic gel and thick cortex. Ca-DPA (5-15% of spore dry weight) binds water to dehydrate the core (water content ~10-50% of vegetative cells), and SASPs (~20% of core proteins) protect DNA. This dehydration forms a gel-like state, but the cortex (peptidoglycan layer) maintains dehydration osmotically rather than forming from these components.​

Option Breakdown

• (A) Both true, [r] correct reason for [a]: Incorrect. While both are true, core dehydration by Ca-DPA and SASPs contributes to heat resistance by stabilizing proteins/DNA, the cortex’s role and other factors (e.g., spore coat, low core water activity) are primary; [r] oversimplifies causation.​

• (B) Both true, [r] not correct reason for [a]: Correct. Assertion and reason are factual, but heat resistance involves multiple mechanisms beyond protoplasm volume reduction alone.​

• (C) Both false: Incorrect, as both statements align with established microbiology.​

• (D) [a] true, [r] false: Incorrect, as reason accurately describes dehydration effects, though not the sole heat resistance cause.​

Correct answer: (B)

Introduction to Endospore Heat Resistance

Endospore heat resistance enables bacteria like Bacillus and Clostridium to endure lethal temperatures for vegetative cells, crucial for CSIR NET Life Sciences. This dormancy form features a dehydrated core with calcium-dipicolinic acid (Ca-DPA) complexes and small acid-soluble spore proteins (SASPs).​

Key Mechanisms of Endospore Survival

• Core Dehydration: Ca-DPA (up to 15% dry weight) chelates calcium, binding water to reduce core water content to ~0.6 g/g dry protein, forming a protective gel that prevents protein denaturation.​

• SASPs Role: These proteins saturate DNA, shielding it from heat/UV damage and providing amino acids for germination; mutants lacking SASPs show reduced viability.​

• Thick Cortex Function: Peptidoglycan cortex maintains dehydration via osmotic activity with counterions, not formed by Ca-DPA/SASPs (core components).​

• Additional Layers: Spore coat and inner membrane low permeability enhance resistance to heat, chemicals, and radiation.​

CSIR NET Assertion-Reason Insight

In exams, both statements are true—endospores resist heat via dehydrated protoplasm—but reason is not the sole cause, as cortex and coats contribute independently.​

Implications for Microbiology

Understanding endospore heat resistance aids food safety (e.g., canning) and biotech, where spores survive autoclaving via these adaptations.​