Question

Psychrotrophic bacteria can grow at very low temperatures owing to the presence of which one of the following in their cell membranes?

• A. Hydrophobic amino acids
• B. Unsaturated fatty acids
• C. Saturated fatty acids
• D. Lipoprotein complexes

Hint:

Membrane Thermal Adaptations:
Cold Growth (Psychrotrophs $4^\circ\text{C}$) $\rightarrow$ HIGH UNSATURATED FATTY ACIDS (cis-double bonds maintain fluidity).
Heat Growth (Thermophiles $65^\circ\text{C}$) $\rightarrow$ HIGH SATURATED FATTY ACIDS (Straight chains maintain rigidity).

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Thermal adaptation of cold-tolerant psychrotrophs: homeoviscous adaptation in Pseudomonas and Bacillus involves desaturase enzyme synthesis of cis-unsaturated fatty acids (mono- and polyunsaturated branched lipids) to maintain plasma membrane fluidity at refrigeration temperatures ($4-7^\circ ext{C}$).
Key Formula or Approach:
\[ \text{Low Temperature } (4^\circ\text{C}) \xrightarrow{\text{Desaturase Induction}} \uparrow \text{cis-Unsaturated Fatty Acids (16:1, 18:1)} \implies \text{Maintains Membrane Fluidity (Prevents Gelation)} \]

Step 2: Detailed Explanation:
In microbial physiology, homeoviscous adaptation, and psychrotrophic bacterial growth:
- Psychrotrophic bacteria (e.g., Pseudomonas fluorescens, Pseudomonas fragi, Bacillus cereus) proliferate in raw and pasteurized milk held at refrigeration storage temperatures ($4^\circ - 7^\circ ext{C}$).
- At near-freezing temperatures, cell membranes normally solidify into a rigid, non-functional gel phase, arresting nutrient transport.
- Psychrotrophs adapt by actively synthesizing high proportions of cis-Unsaturated Fatty Acids (e.g., palmitoleic acid 16:1, oleic acid 18:1) (B) and branched-chain anteiso-fatty acids in their membrane phospholipids.
- The permanent kinks introduced by the cis-carbon-carbon double bonds prevent tight hydrocarbon packing, lowering the membrane phase transition temperature ($T_m$) and maintaining essential membrane fluidity and transport enzyme activity at very low temperatures.

Step 3: Final Answer:
Therefore, psychrotrophic bacteria grow at low temperatures due to Unsaturated fatty acids, matching option (B).