All of the following are characteristics of acid-fast bacteria EXCEPT:
A. Mycolic acids in the cell wall are responsible for acid-fastness and impermeability to aqueous dyes
B. Carbol fuchsin penetrates the cell wall when heated, and acid-alcohol decolorizes non-acid-fast cells but not acid-fast cells
C. Acid-fast bacteria are Gram-positive and retain crystal violet after Gram staining
D. Auramine-rhodamine staining is more sensitive than Ziehl-Neelsen staining for detecting acid-fast bacilli in sputum
Explanation
Acid-Fast Bacteria: Staining and Structure
Definition and Cell Wall Composition
Key Point
Acid-fast bacteria (e.g., Mycobacterium tuberculosis, Mycobacterium leprae) possess a unique cell wall rich in mycolic acids — long-chain, branched fatty acids (C60–C90) that form a waxy, lipid-rich layer.
Why They Are Acid-Fast
High-YieldNEET PG
1.
Mycolic acids create a hydrophobic barrier that repels aqueous dyes
2.
Heat (in Ziehl-Neelsen staining) opens pores in the mycolic acid layer, allowing carbol fuchsin to penetrate
3.
Once inside, the dye is trapped by the waxy lipids
4.
Acid-alcohol (3% HCl in 95% ethanol) cannot remove the dye from acid-fast cells because the mycolic acids re-seal the cell wall
5.
Non-acid-fast cells lose the dye during decolorization
Gram Staining Behavior — The Critical Error
Warning
Acid-fast bacteria are NOT reliably Gram-positive. Although Mycobacterium has a peptidoglycan layer (making it technically Gram-positive in structure), it does NOT retain crystal violet during Gram staining because:
The thick mycolic acid layer is impermeable to crystal violet
Even if crystal violet enters, the lipid-rich wall does not form a stable crystal violet–iodine complex
Result: Acid-fast bacteria appear Gram-negative or Gram-variable on Gram stain, despite having Gram-positive-type peptidoglycan
Clinical Pearl
This is why acid-fast staining (Ziehl-Neelsen, auramine-rhodamine) is the gold standard for Mycobacterium detection, not Gram staining.
Auramine-Rhodamine vs. Ziehl-Neelsen
Table
Feature
Auramine-Rhodamine
Ziehl-Neelsen
Dye
Auramine O (fluorescent)
Carbol fuchsin (red)
Detection
Fluorescence microscopy
Light microscopy
Sensitivity
95–98% (higher)
85–90%
Specificity
99%
99%
Bacilli appearance
Yellow-green on dark background
Red on blue background
Use in TB diagnosis
Preferred for sputum smears
Alternative; good for tissue
High-YieldNEET PG
Auramine-rhodamine is more sensitive because fluorescent dyes are detected with greater ease and less background interference.
Mnemonic: MAAF
Mycolic acids → Acid-fast → Auramine (fluorescent, more sensitive) → Fuchsin (traditional, less sensitive)
Why Option 3 is Correct (The Wrong Statement)
Option 3 states that acid-fast bacteria are Gram-positive and retain crystal violet. This is FALSE:
Acid-fast bacteria do NOT retain crystal violet during Gram staining
They appear Gram-negative or Gram-variable despite having peptidoglycan
This is a classic NEET PG trap: confusing structural classification (Gram-positive peptidoglycan) with staining behavior (Gram-negative appearance)
Prescott's Microbiology 10e Ch 3; Robbins Pathology 10e Ch 8
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