Actinomyces israelii

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Actinomyces israelii


Introduction to bacteria

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Actinomyces israelii


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Actinomyces israelii

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USMLE® Step 1 style questions USMLE

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A 40-year-old man presents to the clinic due to a painless jaw mass that started a few weeks ago. The mass has slowly increased in size and has recently begun producing a yellow discharge. The patient underwent molar extraction a few weeks prior to noticing the mass. Past medical history includes type 1 diabetes mellitus. Temperature is 38.0 ºC (100.4 ºF), pulse is 75/min, and blood pressure is 120/80 mmHg. Physical examination shows a painless mandibular mass with sinus tract formation and yellow purulent discharge. Gram stain and culture of the discharge shows Gram-positive bacteria with yellow sulfur granules. Which of the following is the most likely organism responsible for this patient’s condition?  

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Actinomyces israelii

labs/findings p. 720

oral infections p. 183

pigment production p. 127


Content Reviewers

Actinomyces israelii, or just A. israelii, is a gram-positive rod-shaped bacteria that causes a human infection called actinomycosis.

Calling this bug actinomyces implies that it’s morphologically similar to fungus, because both form microscopic branching filaments (or mycelium), while the word Israelii comes from James Adolf Israel, the German surgeon who first described it.

OK, A. israelii is a rod-shaped gram-positive bacteria, we’ve got that part down, but what this means is that it goes purple when gram-stained.

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

They are anaerobes, meaning they grow better without oxygen, they are also non-motile, and don’t form spores.

But wait… that sounds exactly like Nocardia, 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.

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.

On the other hand, Nocardia has plenty of lipids in its cell wall, so it retains the carbon fuchsin, and it looks red under the microscope, making it an acid-fast bacteria.

Another difference is that A. Israelii is catalase negative, so it doesn’t make an enzyme called catalase, whereas Nocardia is catalase positive.

Finally, A. israelii is cultivated on blood agar, and then incubated in anaerobic conditions.

It’s a slow-glowing microbe, so it takes up to 21 days to form colonies, which look like white round plaques, with some little grooves in the middle, forming a characteristic molar-tooth appearance.

Alright, now, A. Israelii can asymptomatically colonize the oral cavity, particularly the space between the roots and gums, as well as the pharynx, where they gather around the tonsils, and also the intestinal tract, and the urogenital tract, where they coexist with other commensal bacteria.


Actinomyces israelii is a type of bacteria that typically inhabits the mouth, skin, and gastrointestinal tract. However, they can also be found in other body sites, such as the lungs and the genital tract. These bacteria are normally harmless, but they can cause infections if they enter the body through a cut or wound.

A. israelii infections are more common in immunocompromised individuals, such as those with HIV/AIDS or cancer. Symptoms of A. israelii infections depend on the infected organs and can include fever, lower abdominal pain, chronic cough, and unusual vaginal discharge.


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