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Introduction to bacteria
Gram positive bacteria
Gram negative bacteria
Other bacteria



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USMLE® Step 1 style questions USMLE

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A 34-year-old woman comes to the clinic because of difficulty breathing and coughing for the past week. She has been living in a tent in the forest for the past month and these symptoms have slowly progressed. Her medical history includes a kidney transplant six months ago. Her current medications include cyclosporine. Her temperature is 38.3°C (101.5°F), pulse is 96/min, respirations are 22/min, and blood pressure is 102/64 mm Hg. Physical examination shows increased breath sounds on the right on auscultation of the chest. A chest x-ray is performed and shows a consolidation and cavitation in the right upper and middle lung zones. A biopsy is performed and shows a gram-positive, weakly acid-fast, urease-positive, catalase-positive, rod-shaped bacteria forming branching filaments. Which of the following is the most likely cause of this condition?

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Viviana Popa, MD

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.

Finally, Nocardia species grows on routine media, like blood agar or chocolate agar, on selective media, such as modified Thayer-Martin agar or BCYE agar, on mycobacterial media, like Lowenstein Jensen agar and on fungi media, like Sabouraud dextrose agar.

Nocardia forms white, yellow or orange long branching filamentous colonies with a powdered aspect, which usually takes 3 to 5 days to grow, but sometimes it may take as long as 2 to 3 weeks.

Now, Nocardia has a low virulence and therefore nocardiosis occurs more often as an opportunistic infection in immunocompromised people.

It enters the body through inhalation or skin trauma and once inside, it’s ingested by macrophages.

Inside macrophages, it gets wrapped in a vesicle called a phagosome, which normally merges with lysosomes, to form a phagolysosome inside which bacteria are killed by lysosomal enzymes.

However, the cell wall of Nocardia contains a cord factor, or trehalose dimycolate, which is a glycolipid that inhibits phagolysosomal fusion.

Ok, now, another immune system weapon is that neutrophils and macrophages destroy invading bacteria by generating a bunch of toxic oxygen radicals, such as superoxide, which results from oxidative metabolic burst.

But, Nocardia makes two enzymes that break these toxic oxygen radicals.

First, there’s superoxide dismutase which breaks superoxide radicals into oxygen and hydrogen peroxide.

And since hydrogen peroxide is still harmful, Nocardia also makes catalase, which breaks hydrogen peroxide into water.

Now, in pulmonary nocardiosis, the presence of Nocardia triggers an inflammatory response that calls more macrophages to the site of battle.

As more of them make their way there, they surround the bacteria from all sides, and form a granuloma, which prevents it from spreading.