The acid-fast stain was developed in 1882 by Paul Ehrlich to aid in the diagnosis of tuberculosis (TB). Ehrlich observed that dyes were not absorbed by Mycobacterium unless the organism on the slide was heated. Once the dye was absorbed, the cell wall retained the dye even when the smear was washed with a mixture of acid and alcohol (3% hydrochloric acid and 95% ethanol).

The cell wall of bacteria Mycobacterium, Nocardia and parasite Cryptosporidium contains large amounts of glycolipids, especially mycolic acids. This unique cell wall enables them to resist staining and makes them resistant to many common disinfectants. It is the mycolic acid that makes it difficult to absorb dye into the cell. However, once the dye penetrates the cell wall (by using heat or chemicals such as phenol), these glycolipids prevent acid-alcohol from decolorizing the cell. Therefore, these organisms are said to be “acid-fast”. Most bacteria, such as E. coli and Staphylococcus, lack this high mycolic acid content and are easily decolorized by the acid-alcohol. Therefore, these organisms are described as “non-acid-fast”. They absorb the counterstain, methylene blue. The acid-fast stain is therefore a differential stain since it uses two dyes to distinguish two groups of organisms based on the mycolic acid in their cell walls. Heat is used when performing the acid-fast stain to penetrate the mycolic acid in the cell walls.

Several species of Mycobacterium and Nocardia are pathogenic for animals and humans.  Mycobacterium tuberculosis causes TB and is one of the world’s deadliest diseases. According to the CDC, it is estimated that 23% of the global population has latent tuberculosis infections. 1 in 10 will go on to develop the disease. Globally, there are approximately 10 million TB infections per year. There are approximately 1.5 million TB-related deaths worldwide. This disease is the number one cause of death world-wide in patients who have AIDS.

Mycobacterium leprae causes leprosy or Hansen’s disease. Leprosy is rare in the United States with only 150-250 cases per year. In the southern United States, some armadillos are naturally infected with the bacteria that cause Hansen’s disease in people and it may be possible that they can spread it to people. However, the risk is very low and most people who have contact with armadillos are unlikely to get Hansen’s disease.

Worldwide, according to WHO, there are approximately 200,000 new cases of leprosy per year worldwide. India accounts for more than 50% of the global cases. The National Hansen’s Disease Program in Baton Rouge, Louisiana, is the only institution in the United States exclusively devoted to Hansen’s disease. The center functions as a referral and consulting center with related research and training activities. Most American patients are treated under U.S. Public Health Service grants at clinics in major cities or by private physicians.

Nocardia asteroides causes a pulmonary disease called nocardiosis which can resemble tuberculosis. In the United States, around 500-1,000 new cases of Nocardia infection occur annually. An estimated 10-15% of these patients also have HIV infection.  Nocardia can be differentiated from Mycobacterium when stained since Nocardia is usually a branching, filamentous organism and Mycobacterium is a rod-shaped organism.

Cryptosporidium, a protozoan parasite, invades the epithelial cells lining the human digestive tract causing a diarrheal disease called cryptosporidiosis. While this parasite can be transmitted in several different ways, water is a common method of transmission and Cryptosporidium is one of the most frequent causes of waterborne disease among humans in the United States. Diagnosis is achieved through finding oocysts in stool specimens from infected patients. The oocysts are difficult to see with routine stains so a modified acid-fast stain can be used in which stained oocysts appear bright red against a blue-green background.

PRE-ASSESSMENT

PROCEDURE

 Prepare a mixed culture smear

Please watch the video above for a demonstration of how to mix and mash the bacteria in step 8 below. For this Exercise: Use a TSA slant of Mycobacterium and TSA slant of Staphylococcus

1. Clean a glass slide with lens cleaner and a cotton swab. Dispose of the cotton swab in the regular trash. Using a permanent marker, label the top right corner of one slide with “A” for Acid-fast.

2. Add one drop of deionized water to the middle of the slide.

3. Sterilize an inoculating loop and allow it to cool.

4. Remove the cap from the Staphylococcus slant, insert the inoculating loop and obtain a pinhead amount of inoculum.

5. Gently spread the inoculum in the circle of deionized water.

6. Sterilize the inoculating loop and let it cool.

7. Remove the cap from the Mycobacterium slant, insert the inoculating loop and obtain a pinhead amount of inoculum.

8. Please watch the video above for a demonstration of how to vigorously mix and mash the bacteria when making the smear. The waxy nature of Mycobacterium causes them to repel water. Vigorously mix and mash the Mycobacterium and Staphylococcus bacteria against the slide to break up all the large clumps of bacteria. Take your time, this is a critical step to making a good smear for an acid-fast stain. The smear should look like this when the vigorous mixing and mashing is complete.

9. Dry the slide on the slide warmer.

Acid-fast stain procedure

1.  Begin the procedure with the dried slide on the slide warmer. Cover the smear with filter paper and add carbolfuchsin to the filter paper. The filter paper must be wet and saturated with carbolfuchsin.

2. Add 1 to 2 drops of carbolfuchsin when the filter paper looks dry or the edges of the filter paper look gold. The filter paper needs to remain wet and saturated.

3.  After 5 minutes, remove the filter paper(s) from the smear; place it on a paper towel and dispose of it in the trash.

4.  Transfer the slide to the stain rack.

5.  Rinse the smear thoroughly with deionized water. Drain off excess water.

6.  Decolorize the smear by adding acid-alcohol to the smear for 5-10 seconds until the decolorizer running off the smear is clear.

7.  Rinse the smear with deionized water. Drain off the excess water.

8.  Add Loeffler’s methylene blue stain to the smear. Wait 1 minute.

9.  Rinse with deionized water. Blot dry with bibulous paper or paper towel.

10.  Observe the smear under oil immersion. Draw your observations on the worksheet and label an example of the acid fast and non-acid fast organisms.

11.  After you have completed your observation the acid-fast stain, dispose of the slide in the used slide basin.

POST TEST

DISCOVERIES IN MICROBIOLOGY

ELIZABETH BUGIE GREGORY

Elizabeth Bugie Gregory was an American microbiologist and biochemist. In 1944, Gregory, Selman Waksman, and Albert Schatz discovered the antibiotic streptomycin. Gregory was told that it was not important for her name to be on the patent as she would “one day get married and have a family”. Waksman went on the win the Nobel Prize for medicine in 1952 and took full credit for the discovery. Through a court settlement, Schatz was awarded 3% of the royalties for streptomycin and was officially recognized as co-discoverer of streptomycin. Waksman claimed that Gregory was more involved in the discovery than Schatz. Gregory was awarded 0.2% of the royalties for streptomycin.