Actinomyces & Nocardia
Sections
Gram-positive filamentous rods that cause infections in humans.
Actinomyces israelii
A. israelii is anerobic, slow-growing, and has low virulence.
Opportunistic pathogen:
It is a common colonizer of the oral cavity and upper respiratory tract, and is sometimes present in the gastrointestinal and urogenital tracts.
Morphology:
Branching, fungus-like, with colonies, aka, sulfur granules.
Granules comprise bacteria and calcium phosphate, and have a dimpled, molar-like appearance. They do not contain sulfur.
Infections: Actinomyces
Most A. israelii infections occur when the bacteria invade deeper tissues of the oral cavity after trauma or surgery. From there, infections can spread.
Chronic, slow forming granulomatous lesions that become abscesses that drain pus with sulfur granules.
Cervicofacial infection most commonly occurs in the cervicofacial region following dental trauma.
Produces localized swelling, often in the mandibular region resulting in "lumpy jaw."
The abscess may form sinus tracts that erupt on the face.
When infection occurs in other sites, more serious complications can arise.
Central nervous system infection can manifest as a single abscess with headache and focal neurological signs. In the image of an abscess removed from the brain, we see examples of "dust bunny" formation; this reflects aggregation of the filamentous bacteria.
Thoracic infection typically produces nonspecific symptoms such as fever and non-productive cough, and lung abscesses may form.
Abdominal cavity infection can affect any organ, and can produce fever and fatigue. Be aware that digestion problems and inflammation may be mistaken for signs of Crohn's diseases, and masses have been mistaken for tuberculosis and cancerous tumors.
Abdominal cavity infection can affect any organ, and can produce fever and fatigue.
Pelvic actinomycosis has been associated with long-term use of intra-uterine devices; masses can easily be mistaken for tumors.
Prevention:
Includes good oral hygiene, and, in the case of dental procedures, prophylactic antibiotics.
Treatment:
includes drainage or surgical debridement when necessary and administration of penicillin.
Be aware that other species of Actinomcyes are also associated with actinomcyosis.
Nocardia
Includes several species that can infect and cause nocardiosis.
Be aware that Nocardia nomenclature and classification has changed dramatically over the years. As a result, some isolates that were once commonly associated with nocardiosis no longer are. For example, several isolates formerly identified as Nocaria asteroides have been reclassified.
Microbiology:
Weakly acid-fast, with a delicate "beaded" appearance.
Aerobic and catalase-positive, with slow growth.
Unique aerial hyphae, with filaments that grow upward from the colony.
Nocardia are not considered part of the normal human microflora.
Found in soil.
Gain entry to human hosts via inhalation and aspiration.
Individuals with defective cellular immunity are particularly susceptible to infection.
Pathogenic Nocarida have multiple ways to avoid phagocytic destruction, which is key to innate immunity.
The enzymes catalase and superoxide dismustase protect them from the harmful effects of phagocytic reactive oxygen species.
Cord factor: When phagocytosed by macrophages, Nocarida cord factor (aka, trehalose dimycolate) prevents phagosome-lysosome fusion, which means the bacteria avoid bactericidal molecules.
Nocardia can survive and replicate within macrophages, which travel throughout the body.
Infections: Nocardiosis
Overall, Nocardiosis is rare, and manifests as non-specific symptoms.
However, it should be ruled out early to avoid delayed diagnosis and treatment. Fortunately, the bacteria's weakly acid-fast nature and aerial hyphae make it easy to identify.
Lung infection is most common; illness onset is associated with nonspecific symptoms. Lung abscesses and necrosis can develop, and dissemination to other organs can occur.
CNS infection is the most serious form of nocardiosis, and results in abscesses with non-specific symptoms (such as fever and headache); meningitis is possible but infrequent.
Cutaneous infections manifest as granulomas, ulcers, or cellulitis, and may involve nearby lymphatics. Infection can be primary or secondary.
Treatment:
Nocardia response to antibiotics varies, so testing for antimicrobial susceptibility is crucial.
In general, pulmonary infections are treated with trimethoprim-sulfamethoxazole (TMP-SMX) and amikacin;
CNS infections are treated with trimethoprim-sulfamethoxazole and imipenem or cephalosporin.
Prolonged antibiotic treatment is recommended to avoid relapse.
References
Murray, P. R., Rosenthal, K. S., & Pfaller, M. A. Medical microbiology. Philadelphia: Elsevier/Saunders. (2013).
Levinson, W. E. Review of Medical Microbiology and Immunology. 14th Ed. Lange (2016)
Wilson, J.W. (2012). Nocardiosis: Updates and Clinical Overview. Mayo Clin Proc. 87(4): 403-407.
Henderson, B., Ward, J.M., Ready, D. (2000). Aggregatibacter (Actinobacillus) actinomycetemcomitans: a tripple A periodontopathogen. Periodontology. 54:78-105.
Lorskov-Lauritsen, N. (2014). Classification, identification, and clinical significance of Haeomophilus and Aggregatibacter species with host specificity for humans. Clinical Microbiology Reviews. 27(2): 214-240.
Images:
Actinomyces israelii (Wikipedia; author GrahamColm)