Auramine-Rhodamine Fluorochrome Staining

By Acharya Tankeshwar •  Updated: 05/05/22 •  5 min read

Auramine-rhodamine fluorochrome staining also known as “Truant method of staining”, is used to visualize acid-fast bacilli (AFB). Ziehl-Neelsen (hot), Kinyoun (cold) are still widely used methods in developing countries. CDC recommends fluorochrome staining for detecting AFB in primary patient specimens.

Comparison of ZN Staining an Flurochrome Staining
Comparison of ZN Staining and Flurochrome staining

The acid fastness of Mycobacteria is due to their thick cell wall composed of waxes and lipids that have a high content of mycolic acid.

Fluorescent dyes like auramine-rhodamine bind to the mycolic acid present in them and impart bright yellow or orange fluorescence against a greenish background when viewed using a fluorescent microscope. It is also used to stain all acid-fast organisms including the sporozoan parasites.

Principle

The fluorochrome dye, auramine-rhodamine, forms a complex with mycolic acids found in the acid-fast cell wall of organisms which resist decolorization by acid-alcohol. The counterstain, potassium permanganate, renders tissue and its debris nonfluorescent, thus reducing the possibility of artifacts. The cells visualized under ultraviolet light appear bright yellow or reddish-orange.

Materials

Reagents: 

Others:

  1. Slide: use only new, unscratched, and clean slides; using old, scratched, or dirty slides can lead to erroneous results.
  2. Identifier: Properly label each slide using graphite pencils or use a diamond or tungsten carbide stylus.
  3. Slide racks
  4. Bunsen burner

Procedure

1.Preparation of Smear:

  1. For Sputum: Using a piece of stick, transfer a purulent part of the sputum (containing any yellow caseous material), to a slide and make a thin smear. An area of approximately ½ by 1 inch (or 2-cm square) is recommended. Spread the smear using circular movements. Allow to air dry.
Fig: Smear Thickness
Fig 1: Smear Thickness

Note: Be sure to prepare smears of suitable thickness. Smears that are too thick may flake during staining and may be difficult to decolorize. Acid-fast organisms that might be present may be obscured. Smears that are too thin may not contain enough sample.

Either condition–too thick or too thin–can lead to erroneous results, particularly false negatives. Here (image 1) the smear in the center is of the proper thickness. Hold a smear about 3 to 4 inches over news-print, if you are just able to read the print, the smear is of proper thickness.

  1. For urine: Make a smear of the deposit from three centrifuged early morning urine sediments.

Allow to air dry and heat fix the specimen.  The use of an electric slide warmer is usually the preferred method for heat-fixing smears.  An alternate method of heat-fixing is to pass the dried slide, smear facing upward, 2 to 3 times through the blue cone of a burner flame.

Fig. Heat Fixation of smear (Upper: using electric heater, lower: using burner)
Fig. Heat Fixation of smear (Upper: using electric heater, lower: using burner)
  1. Staining Method

Result and Interpretation

Patterns for Examining the slide
Patterns for Examining the slide

Number of Fields to Examine: 

The minimum number of fields to examine before reporting a smear as negative for acid-fast organisms.

Final magnification (the objective lens magnification multiplied by the eyepiece magnification) Vs. Number of slides

  1. 200x: 30
  2. 250x: 30
  3. 400x: 55
  4. 450x: 70

Reporting of smears

Examining and Reporting Acid Fastness
Examining and Reporting Acid fastness

Advantages

  1. Faster screening of smears than with ZN- Smears can be examined rapidly using a 40x objective or 25x objective. This increases the chances of detecting AFB especially when they are few.
  2. ~10% more sensitive than ZN.
  3. Does not require the use of oil immersion.
  4. No heat is required for staining.
Fig:Fluorochrome stained smear showing numerous green acid fast bacilli
Fig: Fluorochrome stained smear showing numerous green acid-fast bacilli

Limitations of Fluorochrome Staining

  1. A fluorescent microscope is required, which may be readily unavailable and also costly (limitation for developing countries)
  2. A positive staining reaction provides presumptive evidence of the presence of mycobacteria. A negative staining reaction does not indicate that the specimen will be culturally negative. Therefore, cultural methods must be employed.
  3. Reagents like auramine-rhodamine are possible carcinogens, acid –alcohol and potassium permanganate is also a strong irritant to skin, eyes, and respiratory system. Caution is required while handling and staining using such reagents.
  4. Most strains of rapid growers may not appear fluorescent.
  5. It is recommended that all negative fluorescent smears be confirmed with Ziehl-Neelsen stain; at least 100 fields should be examined before being reported as negative.
  6. Excessive exposure to the counterstain may result in a loss of brilliance of the fluorescing organism.
  7. Stained smears should be observed within 24 hours of staining because of the possibility of fluorescence fading.

Download
Power points presentation ” Use of Fluorochrome Staining for Detecting Acid-fast Mycobacteria” by CDC
Note: Most of the above images were reproduced from this PPT Slide of CDC.

Acharya Tankeshwar

Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. As an asst. professor, I am teaching microbiology and immunology to medical and nursing students at PAHS, Nepal. I have been working as a microbiologist at Patan hospital for more than 10 years.