Q.30 The acid – fastness of mycobacteria is due to :
1. Presence of mycolic acids in cell wall
2. Presence of peptidoglycan in the cell wall
3. Presence of glycoproteins in cell wall
4. Presence of glycolipids in cell wall

The acid-fastness of mycobacteria is due to the presence of mycolic acids in their cell wall, making option 1 the correct answer. This property allows them to retain stains like carbol fuchsin even after acid-alcohol decolorization.

Question Breakdown

This MCQ targets microbiology concepts, specifically mycobacterial cell wall structure and staining, key for exams like GATE Life Sciences where acid-fast staining (Ziehl-Neelsen) is a standard topic.

Option Analysis

• Option 1: Presence of mycolic acids in cell wall
  Correct. Mycolic acids are long-chain (C70-C90) 3-hydroxy fatty acids covalently linked to arabinogalactan-peptidoglycan in the mycobacterial cell wall. They form a waxy, hydrophobic barrier that traps lipid-soluble stains and resists acid-alcohol removal, enabling acid-fastness.

• Option 2: Presence of peptidoglycan in the cell wall
  Incorrect. Peptidoglycan provides rigidity in all bacteria but lacks the waxy lipids needed for acid resistance; mycobacteria’s peptidoglycan is modified but not the primary factor.

• Option 3: Presence of glycoproteins in cell wall
  Incorrect. Glycoproteins are not key cell wall components in mycobacteria; they occur elsewhere (e.g., host cells) but don’t confer acid-fastness.

• Option 4: Presence of glycolipids in cell wall
  Incorrect. Glycolipids like lipoarabinomannan modulate immunity but contribute minimally to the hydrophobic barrier compared to mycolic acids.

Introduction to Acid-Fastness

The acid-fastness of mycobacteria stems from mycolic acids in their cell wall, creating a impermeable waxy layer vital for diagnosing tuberculosis via Ziehl-Neelsen staining. This unique trait distinguishes pathogens like Mycobacterium tuberculosis.

Mechanism of Mycolic Acids

Mycolic acids embed in the outer membrane, softening under heat to allow carbol fuchsin penetration but preventing acid-alcohol washout due to hydrophobicity. Mutants lacking them (e.g., kasB deletion) lose acid-fastness.

Clinical Relevance

This property aids rapid TB detection; non-acid-fast bacteria decolorize easily. In exams, link mycolic acids to both virulence and staining resistance.