Actinomyces israelii is a gram (positive/negative) , anaerobic, filamentous bacteria.
USMLE® Step 1 style questions USMLE
USMLE® Step 2 style questions USMLE
A 45-year-old man comes to the clinic because of purulent abscesses around his mandible. He has been homeless for several years, and says that the lesions have grown steadily in the past months. It is now affecting his ability to swallow. Tissue biopsy shows granulation tissue with rare yellow granules and filaments that are not acid fast. Which of the following organisms is the most likely causative agent?
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.
A. Israelii doesn’t cause any trouble to the host, as long as the host’s immune system keeps them in check, restricting their growth, and preventing them from invading deep into tissues.
Another reason is poor oral hygiene, which allows the bacteria to proliferate in the oral cavity, increasing the chance of an infection.
Now, actinomycosis can present in a number of ways.
The most common form is cervicofacial actinomycosis, which affects the mouth, jaw, and neck region.
It typically starts with a breach in the oral mucosa, which can happen when there’s trauma to it, like when having dental work done, or if there’s trauma to local tissues, like after craniofacial surgery.
The bacteria can then take advantage of the injured epithelial barrier to get into deep tissues, triggering an inflammatory process, which is typically followed by formation of abscesses - pockets of pus within infected tissues.
The abscesses may fistulize, meaning they create a sinus tract, which is like a pipe that drains the pus out through the skin.
In severe cases, the infection can spread deep into jawbones resulting in osteomyelitis of the maxilla or mandible, and it can sometimes spread to the middle ear resulting in actinomycosis otitis media.