Acid Fast Bacteria

Acid-fast bacteria, also known as acid-fast bacilli or simply AFB, are a group of bacteria sharing the characteristic of acid fastness. Acid fastness is a physical property that gives a bacterium the ability to resist decolorization by acids during staining procedures. This means that once the bacterium is stained, it cannot be decolorized using acids routinely used in the process. This important and unique feature of certain bacteria gives the ability to classify and detect them using relatively easy laboratory procedures such as microscopy. Bacteria displaying acid fastness include:

1. Genus Mycobacterium – M. leprae, M. tuberculosis, M. smegmatis, M. Avium complex, M. kansasii.

2. Genus Nocardia – N. brasiliensis, N. cyriacigeorgica, N. farcinica, and N. nova.

Acid fastness can also be attributed to other structures not classified as bacteria. These include:

1. Bacterial endospores

2. Head of sperm

3. Cryptosporidium parvum

4. Isospora belli

5. Cyclospora cayetanensis

6. Taenia saginata eggs

7. Hydatid cysts

8. Sarcocystis

9. Nuclear inclusion bodies in lead poisoning

Even though acid fastness can be attributed to many different bacteria, correlation with history makes it a fairly unique characteristic of M. tuberculosis in clinical practice. This makes acid-fast staining sensitive and specific, provided clinical correlation is part of the equation. This writing will focus on the acid-fast bacteria M. tuberculosis. The diagnosis of M. tuberculosis using this characteristic is referred to as TB microscopy, acid-fast smear microscopy, and direct sputum near microscopy. Even though the use of highly advanced molecular diagnostic tests has come into play, the value of this staining technique cannot be overstated, especially for low and middle-income countries.