Microorganisms PYQs – Biology, SSC & Railway

Answer: Option 4 – Pseudomonas — Nitrification (incorrect pairing).
• *Pseudomonas* carries outdenitrification(nitrate → nitrogen gas), not nitrification. The other three pairings are correct.

Topic: Leavening of Bread
Yeast (Saccharomyces cerevisiae) breaks down sugars in dough.
Fermentation Process:
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
(Glucose → Ethanol + Carbon dioxide)
CO₂ gas forms bubbles, making bread rise and fluffy.
Ethanol evaporates during baking.
Answer: Option 3 → Fermentation

Correct Answer: Option 3 – Dehydration
Dehydration (drying/desiccation) preserves food by removing water content, thereby reducing the water activity (aw) below the level needed for microbial growth. Most bacteria require aw > 0.91, yeasts > 0.88, and moulds > 0.70 to grow. By removing moisture through sun-drying, spray drying, freeze-drying, or hot air drying, microbial growth is inhibited. Examples: dried fruits, jerky, powdered milk, spices.
Option 1 (Refrigeration): Preserves by lowering temperature, slowing metabolic rates and microbial growth — not primarily through moisture removal.
Option 2 (Canning): Preserves by heat sterilization (killing microbes) and vacuum sealing to prevent recontamination — not primarily through moisture reduction.
Option 4 (Pasteurization): Uses controlled heat treatment (e.g., 72°C for 15 seconds for milk) to kill pathogenic microorganisms — not moisture-based preservation.

Correct Answer: Option 1 – This is the INCORRECT statement
Option 1 is INCORRECT: Gram positive bacteria have a THICK peptidoglycan cell wall, which actually RETAINS the crystal violet-iodine complex (they stain purple/violet). The statement says they are 'unable to retain' crystal violet — this is the opposite of the truth.
Option 2 is CORRECT: Gram negative bacteria DO have a thin peptidoglycan layer (plus an outer membrane) and CANNOT retain crystal violet after decolorization.
Option 3 is CORRECT: Gram's iodine acts as a mordant, forming an insoluble crystal-violet-iodine (CV-I) complex that gets trapped inside the thick cell wall of Gram positive bacteria.
Option 4 is CORRECT: Gram negative bacteria lose the crystal violet after decolorization and take up the red/pink safranin counterstain, appearing red/pink under the microscope.