Mycobacteria

Mycobacteria are very thin, rod-shaped, bacteria. They are slow growing and can be stained with an acid-fast stain. Mycobacteria don’t stain well with a gram stain due to their highly impenetrable hydrophobic cell wall. The acid-fast stain starts with a lipid soluble dye, commonly carbol fuchsin, that can penetrate the cell wall of the mycobacteria. Then an acid decolorizer is used and is ineffective on the “acid-fast” mycobacteria. The name doesn’t really make much sense and “acid fast” simply means the acid alcohol decolorizer is not effective on mycobacteria.

The major groupings of mycobacteria are Mycobacteria tuberculosis complex and Nontuberculosis Mycobacteria (NTM), and are simply separated by whether or not they cause tuberculosis. The list of NTM is long, and the M. tuberculosis group has four species:

M. tuberculosis
M. bovis
M. bovis BCG
M. africanum

For mycobacteria, there are a few differing characteristics that can distinguish different species, and there are a few common chemical tests that are run. The table below will help you focus on the most common species, characteristics, and tests you need for a basic understanding.

Mycobacteria	Fast growth on MacConkey	Niacin +	Nitrate reduction	Optimal growth 30C	Smooth or rough	Associated with AIDS patients
M. fortuitum	X		X
M. chelonae	X
M. tuberculosis		X	X		R
M. marinum				X
M. ulcerans				X
M. kansasii			X
M. gordonae					S
M. avium complex						X

Study Tip
Use mnemonics to remember how to group different mycobacteria species, for example “Fort chelonae was built fast on Mac.” What does this sentence tell me? It tells me that M. fortuitum and M. chelonae are rapid growers on MacConkey agar. Let’s try one more: “Mari is 30 and has an Ulcer on her skin.” This tells me that M. marinum and M. ulcerans have an optimal growth temperature of 30C, and are associated with skin infections. (Not necessarily an ulcer on the skin)

M. tuberculosis and niacin – M. tuberculosis lacks the enzyme to convert niacin to niacin ribonucleotide and for that reason niacin accumulates in M. tuberculosis.

In addition to these characteristics, the ability of mycobacteria to produce pigment after being grown in culture can be an important way to make a distinction. There are three types of classification based on whether or not light was needed to produce the pigment: photochromogen, scotochromogen, and non-photochromogen. All of these groups are NTM (Nontuberculosis Mycobacteria).

Photochromogens are slow growing NTM whose colonies become pigmented when exposed to light. Important species to remember are M. marinum and M. kansasii.

Study Tip
Use the mnemonic "Take photos of Mari in Kansas" to remember photochromogens.

Scotochromogens are slow growing NTM whose colonies become pigmented in the dark or light. Important species to remember are M. szulgai, M. scrofulaceum, M. gordonae, and M. flavescens.

Study Tip
Use the mnemonic "SSG flav" to remember scotochromogens (If the species starts with S or G it’s most likely a scotochromogen, flav distinguishes flavescens from fortuitum).

Non-photochromogens are slow growing NTM whose colonies produce no pigment in the light or dark. If the organism is not in the photchromogen or scotochromogen group, there’s a good chance it is a non-photochromogen.