Rhodamine‑Auramine stain is a fluorochrome staining technique that combines auramine O and rhodamine B dyes to detect acid‑fast bacteria such as mycobacteria by fluorescence microscopy.
Principle and Method
The auramine–rhodamine stain, sometimes called the Truant fluorochrome stain, exploits the affinity of acid‑fast organisms for basic phenolic dyes. Mycobacteria and other acid‑fast organisms have cell walls rich in mycolic acids and other lipid components that resist decolorization by acid alcohol. In this technique, smears are heat fixed and flooded with a staining solution containing auramine O and rhodamine B. The fluorochromes penetrate the waxy cell wall and bind to mycolic acid. After decolorization with a mild acid–alcohol solution to remove nonspecifically bound dye and a rinse, the slide is counterstained, often with potassium permanganate or a dark quencher. Under ultraviolet illumination, acid‑fast bacilli fluoresce bright yellow or orange against a dark background. Non‑acid‑fast cells and debris appear faintly red or are quenched by the counterstain.
Applications and Advantages
The rhodamine–auramine stain is widely used as a screening tool for Mycobacterium tuberculosis in clinical specimens such as sputum because it is more sensitive than conventional Ziehl–Neelsen or Kinyoun acid‑fast stains and allows rapid scanning of smears at lower magnification. It also detects other acid‑fast organisms, including Mycobacterium avium complex, Mycobacterium kansasii and Nocardia species. Positive smears should be confirmed by culture or nucleic acid amplification because the stain is not specific for the Mycobacterium tuberculosis complex. The requirement for a fluorescent microscope is a limitation for some laboratories, but LED fluorescence microscopes have made the technique more accessible. The method reduces eye fatigue and improves detection in settings with high workloads.
The rhodamine–auramine stain provides a sensitive, rapid means of screening for acid‑fast bacteria. Its reliance on fluorescence microscopy and need for confirmatory testing should be considered when interpreting results.
Related Terms: Acid‑fast stain, Ziehl‑Neelsen stain, Auramine O, Mycobacterium tuberculosis, Fluorescence microscopy