Partial acid-fastness: Nocardia species resist decolorization with which concentration of sulphuric acid in modified acid-fast staining?

Introduction / Context:
Nocardia are aerobic, branching, filamentous, Gram-positive bacteria that show partial (weak) acid-fastness due to intermediate mycolic acids in their cell wall. Distinguishing Nocardia from fully acid-fast Mycobacterium is important in respiratory and cutaneous infections.

Given Data / Assumptions:

• Modified Ziehl–Neelsen or Kinyoun methods are used for weak acid-fast organisms.
• Question focuses on the decolorizer strength in which Nocardia retains stain.
• Classical teaching: Nocardia resists decolorization with ~0.5–1% sulphuric acid, not with stronger acids used for mycobacteria.

Concept / Approach:
Mycobacteria withstand strong acid-alcohol or 20% sulphuric acid decolorizers and remain acid-fast. Nocardia, with shorter-chain mycolic acids, are only weakly acid-fast and typically retain carbol fuchsin when a mild acid (around 0.5% sulphuric acid) is used. With higher concentrations (5–10%), they generally lose the stain.

Step-by-Step Solution:
Stain smear with carbol fuchsin (with or without heat). Decolorize using mild acid: 0.5% H2SO4 for Nocardia. Counterstain (e.g., methylene blue). Observe partially acid-fast branching filaments retaining red stain.

Verification / Alternative check:
Parallel smears decolorized with stronger acid will typically show loss of acid-fastness for Nocardia but preserved acid-fastness for Mycobacterium, supporting the difference.

Why Other Options Are Wrong:

• 5%, 8%, 10% H2SO4: Too strong; Nocardia are decolorized at these concentrations.
• 1% HCl in alcohol: Standard acid-alcohol for mycobacteria; again, Nocardia usually decolorize.

Common Pitfalls:
Over-decolorization may falsely classify Nocardia as non–acid-fast; always specify the decolorizer concentration in your report or protocol.

Final Answer:
Nocardia retain stain with 0.5% sulphuric acid decolorization.