Actinomyces israelii

Definitions & Key takeaways

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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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.
Problems arise, however, when the immune system becomes weak for some reason, like with an HIV infection, or underlying malignancy or diabetes mellitus.
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.
There can also be respiratory actinomycosis, and this typically happens following trauma to the mucosa of the respiratory tract.
This can be after aspiration of a foreign body like a fishbone, or a mucosa laceration by an endotracheal stent in an intubation process.
When the infection affects the larynx, it’s called laryngeal actinomycosis. Sometimes the infection goes down to affect the bronchi to cause bronchial actinomycosis, or pulmonary actinomycosis if the infection makes it to the lung tissue.
Abdominal actinomycosis can also occur, and is especially common in people who recently had trauma to the bowels, like in perforated appendicitis, or those who underwent abdominal surgery, specifically on the intestines.
This form of actinomycosis mainly affects the appendix and the ileocecal region, and it may result in the formation of a mass of granulation tissue that mimics a slow growing tumor.
In rare cases, this infection can affect the genitourinary tract, and is common in females who have had an intrauterine device - IUD in place for a long period of time - like over five years.
When this happens, it can result in pelvic inflammatory disease, an infection affecting the cervix, uterine cavity, fallopian tubes, and ovaries.
Symptoms of actinomycosis usually start with low-grade fever, with other symptoms depending on which part of the body is affected.
For example, with cervicofacial actinomycosis, there can be slowly growing masses, which may associate a characteristic yellow pus discharge.
With respiratory actinomycosis, there may be chest pain, a cough, and the sputum may be bloody and contain yellow pus. With abdominal actinomycosis, there’s non-specific symptoms like abdominal discomfort and changes in bowel habits.
Finally, with genitourinary actinomycosis, there may be lower abdominal pain and vaginal discharge. Diagnosis is suspected when a Gram stain of samples taken from the site of infection show purple branching rods suggestive of A.
israelii. Cultures are done afterwards to confirm the diagnosis.
Treatment is done with antibiotics such as penicillin G, or doxycycline and clindamycin in case of allergy to penicillin G.
Surgical debridement may be necessary in case of extensive disease. Also, implants such as IUDs should be removed in case of IUD-associated genitourinary actinomycosis.
All right, as a quick recap… Actinomyces israelii is a rod-shaped, gram-positive, non-motile, non-spore forming, catalase negative bacteria that grows in branching filaments.
It asymptomatically colonizes the gut in healthy individuals, but it can cause actinomycosis in immunocompromised patients.
Symptoms depend on affected systems, but they generally include low-grade fever, fistulizing abscesses with yellow pus and granulation tissues that mimic a slow-growing tumor.
Diagnosis is suspected based on Gram-staining, but confirmation requires cultures. Treatment is done with penicillin G, or doxycycline and clindamycin, and surgical debridement may be done for extensive disease.