Actinomyces and Nocardia
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Direct answer
Actinomyces and Nocardia are branching, filamentous Gram-positive bacteria that cause chronic suppurative infections. Actinomyces israelii causes actinomycosis, which forms draining sinus tracts that discharge yellow sulphur granules, and it is anaerobic or capnophilic and catalase negative. Nocardia causes nocardiosis, mainly in immunocompromised people, and it is aerobic, catalase positive and partially acid-fast.
What you must remember
- Actinomyces israelii: a non-acid-fast Gram-positive branching anaerobe or capnophile that is normally present in the mouth, gut and female genital tract.
- Sulphur granules: yellow clusters in pus from sinus tracts, which show radiating club-shaped filaments on microscopy and are the hallmark of actinomycosis.
- Sites of actinomycosis: cervicofacial disease is commonest, often after dental trauma, followed by thoracic, abdominal and pelvic disease.
- Device link: pelvic actinomycosis is classically associated with intrauterine contraceptive devices.
- Nocardia: an aerobic, beaded, branching Gram-positive filament that is weakly acid-fast on a modified Ziehl-Neelsen stain.
- Nocardiosis: pulmonary infection, brain abscess and cutaneous disease, with pulmonary disease most often seen in people on corticosteroids or after transplantation.
- Source: Nocardia lives in soil and decaying vegetation, whereas Actinomyces is a commensal of the human body.
- Treatment: high-dose penicillin for prolonged periods with drainage for actinomycosis, and co-trimoxazole for nocardiosis.
Telling the two apart in the laboratory
Actinomyces and Nocardia look alike in tissue and both branch on a Gram stain, but their oxygen requirement, catalase reaction and acid-fastness separate them. The table lists the features that help at the bench and in the exam.
| Feature | Actinomyces israelii | Nocardia species |
|---|---|---|
| Oxygen requirement | Anaerobic or capnophilic | Strictly aerobic |
| Catalase | Negative | Positive |
| Acid-fastness | Not acid-fast | Partially acid-fast |
| Granules in pus | Sulphur granules | Not typical |
| Main disease | Cervicofacial actinomycosis | Pulmonary and brain nocardiosis |
| Drug of choice | High-dose penicillin | Co-trimoxazole |
Catalase and oxygen requirement are the quickest bench tests, and the stain then confirms the result. The drug choice follows directly from the identification, because penicillin is used for Actinomyces while Nocardia is treated with sulphonamide-based therapy. Both organisms can resemble fungi in tissue sections, so culture and special stains are needed before the diagnosis is settled.
In an exam stem, a draining jaw swelling with sulphur granules points to Actinomyces, whereas an immunocompromised patient with lung nodules and beaded filaments points to Nocardia. Pus sent for anaerobic culture as well as aerobic culture avoids missing Actinomyces.
Where the exam tries to catch you
These two organisms appear together in questions on filamentous bacteria, so the granule, the catalase result and the host situation usually decide the answer.
- Trap: calling Nocardia acid-fast and therefore a mycobacterium Correct: Nocardia is only partially acid-fast and grows on routine media within days, unlike slow-growing mycobacteria.
- Trap: treating actinomycosis with a short antibiotic course Correct: actinomycosis needs prolonged high-dose penicillin, often for months, along with drainage of abscesses to prevent relapse.
- Trap: expecting sulphur granules in a typical Nocardia infection Correct: sulphur granules are the characteristic finding of actinomycosis, not nocardiosis.
Frequently asked questions
Why are sulphur granules characteristic of Actinomyces israelii?
Sulphur granules are yellow clusters of bacterial filaments found in pus from draining sinus tracts in actinomycosis. Microscopy shows radiating club-shaped filaments at the edge of each granule, which makes the finding diagnostic of Actinomyces israelii infection.
Why is Nocardia described as partially acid-fast?
Nocardia has a mycolic acid-containing cell wall that retains carbol fuchsin only partially, so it is described as weakly or partially acid-fast. A modified Ziehl-Neelsen stain with weaker acid decolourisation shows this feature, which helps to separate Nocardia from Actinomyces.
Which drug is used for nocardiosis?
Co-trimoxazole is the first-line drug for nocardiosis caused by Nocardia species, usually given for many months in pulmonary and disseminated disease. Intravenous therapy is used in severe or cerebral disease, per current guidance.
Which device is linked to pelvic actinomycosis?
Intrauterine contraceptive devices are the classic device associated with pelvic actinomycosis, where Actinomyces israelii colonises the device and forms pelvic abscesses. Removal of the device together with prolonged penicillin is the usual management of this form.
How can actinomycosis be told apart from tuberculosis?
Actinomycosis is separated from tuberculosis by its sulphur granules, its anaerobic or capnophilic growth and its lack of acid-fastness. Tuberculosis is acid-fast and takes weeks to grow on culture, whereas Actinomyces grows within days in suitable conditions.