Nocardia

Nocardia is a genus of Gram-positive branching filamentous rods that are often found in soil.

There are over 80 species of Nocardia and around 30 of them causes disease in humans, and the most notable ones are Nocardia asteroides, Nocardia brasiliensis, Nocardia cyriacigeorgica, Nocardia farcinica and Nocardia nova.

Nocardia causes a disease called nocardiosis which has three major forms - pulmonary, cutaneous and disseminated.

OK, Nocardia is a rod-shaped Gram-positive bacteria, we’ve got that part down, and this means it goes purple when Gram-stained.

When there’s many of them, they arrange themselves in the shape of purple branching filaments.

They are obligate aerobes, meaning they need oxygen to grow, they are also non-motile, and don’t form spores.

But wait… that sounds exactly like Actinomyces israelii, another group of rod shaped, gram-positive, filamentous bacteria with a lot of other similar features.

To distinguish them, an acid-fast stain, also called Ziehl-Neelsen stain is done.

With this test, a red dye called carbon fuchsin, binds to lipids in the cell wall, coloring them red.

Then alcohol is applied to wash out any dye that hasn’t colored bacteria, and a second dye, methylene blue, is applied.

Now, Nocardia is a weak acid-fast bacteria which means that a less concentrated solution of alcohol is needed during staining and that’s because the mycolic acids in its cell wall have intermediate-length.

So, because it has plenty of lipids in its cell wall, it retains the carbon fuchsin, and it looks red under the microscope, making it an acid-fast bacteria.

On the other hand, in bacteria who don’t have a lot of lipids in their cell wall, like A. Israelii, all the red dye is washed off by alcohol, so it looks blue under the microscope, making it a non-acid-fast bacteria.

Also, Nocardia can be visualized with auramine-rhodamine stain using fluorescence microscopy, which can show a reddish-yellow fluorescence.

This stain is not as specific as Ziehl-Neelsen stain, so it has more false-positive results, but it’s more sensitive, so it has less false-negative results, and it’s also inexpensive.

So the auramine-rhodamine stain is often used for screening.

Another difference is that Nocardia is catalase positive, so it makes an enzyme called catalase which converts hydrogen peroxide to water and oxygen, and also urease positive which means that it produces an enzyme called urease that can break urea into ammonia and carbon dioxide, whereas A. israelii is catalase and urease negative.