Which of the following bacteria is acid-fast on staining? (Consider partial/weak acid-fastness with modified methods.)

Introduction:
Acid-fast staining detects organisms with mycolic acid–rich cell walls. This question probes recognition of genera that exhibit acid-fastness, including weak or partial acid-fastness observed with modified stains.

Given Data / Assumptions:

• Standard Ziehl–Neelsen and modified Kinyoun methods are considered.
• We focus on genus-level traits relevant to clinical microbiology.
• “Acid-fast” may include weakly acid-fast organisms.

Concept / Approach:
Nocardia species contain intermediate-length mycolic acids and are classically weakly acid-fast by modified staining. Actinomyces and Streptomyces lack this property, and Corynebacterium are not acid-fast (except C. diphtheriae metachromatic granules, which is unrelated to acid-fastness).

Step-by-Step Solution:
Recall the role of mycolic acids in retaining carbol fuchsin dye against acid-alcohol decolorization. Identify Nocardia as weakly acid-fast with modified Ziehl–Neelsen staining. Exclude non–acid-fast genera: Actinomyces, Streptomyces, Corynebacterium. Select Nocardia as the correct choice.

Verification / Alternative check:
Clinical labs routinely apply a modified acid-fast stain for nocardiosis workups, confirming this characteristic.

Why Other Options Are Wrong:

• Actinomyces: Not acid-fast; Gram-positive filamentous but lacks mycolic acids.
• Streptomyces: Soil actinomycete; Gram-positive, non–acid-fast.
• Corynebacterium: Non–acid-fast; irregular Gram-positive rods.

Common Pitfalls:
Confusing filamentous morphology (Actinomyces/Streptomyces) with acid-fastness leads to misclassification.

Final Answer:
Nocardia is acid-fast (weakly) on modified staining.