Review of Medical Microbiology and Immunology, 13th Edition

22. Actinomycetes



Actinomyces israelii

Nocardia asteroides

Self-Assessment Questions

Summaries of Organisms

Practice Questions: USMLE & Course Examinations


Actinomycetes are a family of bacteria that form long, branching filaments that resemble the hyphae of fungi (Figure 22–1). They are gram-positive, but some (such as Nocardia asteroides) are also weakly acid-fast rods (Table 22–1).


FIGURE 22–1 Nocardia asteroides—Gram stain. Arrow points to area of filaments of gram-positive rods. (Figure courtesy of Dr. Thomas Sellers, Emory University, Centers for Disease Control and Prevention.)

TABLE 22–1 Actinomycetes




A. israelii causes actinomycosis.

Important Properties & Pathogenesis

A. israelii is an anaerobe that forms part of the normal flora of the oral cavity. After local trauma such as a broken jaw or dental extraction, it may invade tissues, forming filaments surrounded by areas of inflammation.

Clinical Findings

The typical lesion of actinomycosis appears as a hard, nontender swelling that develops slowly and eventually drains pus through sinus tracts (Figure 22–2). Hard, yellow granules (sulfur granules) composed of a mass of filaments are formed in pus.


FIGURE 22–2 Actinomycosis. Note inflamed lesion with small sinus tract opening anterior to right ear. Yellowish “sulfur granule” can be seen at the opening. (Figure courtesy of Dr. Thomas F. Sellers, Public Health Image Library, Centers for Disease Control and Prevention.)

In about 50% of cases, the initial lesion involves the face and neck; in the rest, the chest or abdomen is the site. Pelvic actinomycosis can occur in women who have retained an intrauterine device for a long period of time. A. israelii and Arachnia species are the most common causes of actinomycosis in humans. The disease is not communicable.

Laboratory Diagnosis

Diagnosis in the laboratory is made by (1) seeing gram-positive branching rods, especially in the presence of sulfur granules; and (2) seeing growth when pus or tissue specimens are cultured under anaerobic conditions. Organisms can be identified by immunofluorescence. There are no serologic tests.

Treatment & Prevention

Treatment consists of prolonged administration of penicillin G, coupled with surgical drainage. There is no significant resistance to penicillin G. No vaccine or prophylactic drug is available.



N. asteroides causes nocardiosis.

Important Properties & Pathogenesis

Nocardia species are aerobes and are found in the environment, particularly in the soil. In immunocompromised individuals, they can produce lung infection and may disseminate. In tissues, Nocardia species are thin, branching filaments that are gram-positive on Gram stain. Many isolates of N. asteroides are weakly acid-fast (i.e., the staining process uses a weaker solution of hydrochloric acid than that used in the stain for mycobacteria). If the regular-strength acid is used, they are not acid-fast.

Clinical Findings

N. asteroides typically causes either pneumonia, lung abscess with cavity formation, lung nodules, or empyema. From the lung, the organism can spread to various organs, notably the brain, where it causes brain abscess. Disease occurs most often in immunocompromised individuals, especially those with reduced cell-mediated immunity. Nocardia brasiliensis, a different species of Nocardia, causes skin infections in the southern regions of the United States and mycetoma, usually in tropical regions.

Laboratory Diagnosis

Diagnosis in the laboratory involves (1) seeing branching rods or filaments that are gram-positive (Figure 22–1) or weakly acid-fast in an acid-fast stain and (2) seeing aerobic growth on bacteriologic media in a few days.

Treatment & Prevention

Treatment is with trimethoprim-sulfamethoxazole. Surgical drainage may also be needed. Occasional drug resistance occurs. No vaccine or prophylactic drug is available.


1. Your patient is a 75-year-old woman with fever and a painful nodule on her forearm. She also has a nonproductive cough that she says is worse than her usual smoking-related cough. She is taking high-dose corticosteroids (prednisone) for an autoimmune disease. Chest X-ray reveals a nodular lesion in the right upper lobe. A biopsy of the nodule on her arm was obtained. Gram stain of the specimen showed filaments of gram-positive rods. The rods were also weakly acid-fast. Regarding the causative organism, which one of the following is most accurate?

(A) Culture of the organism should be done under anaerobic conditions.

(B) The natural habitat of the organism is the soil.

(C) It produces an exotoxin that inhibits protein synthesis by ADP-ribosylation.

(D) Sulfur granules are often seen in the skin lesion.

(E) The vaccine against this organism contains the capsular polysaccharide as the immunogen.

2. Your patient is a 20-year-old man who was in a fist fight in a bar about 3 weeks ago. He took a punch that broke his left second molar. He now has a 3-cm inflamed area on the skin overlying the broken tooth that is draining pus. A Gram stain of the pus reveals gram-positive filamentous rods. The rods did not appear red in the acid-fast stain. Regarding the causative organism, which one of the following is most accurate?

(A) Infections caused by this organism occur primarily in the Ohio and Mississippi River Valley area.

(B) The natural habitat of the organism is the soil.

(C) This organism is resistant to both penicillins and aminoglycosides.

(D) Sulfur granules are often seen in the pus located at the orifice of the sinus tract in the skin lesion.

(E) The vaccine against this organism contains a toxoid as the immunogen.


1. (B)

2. (D)


Brief summaries of the organisms described in this chapter begin on page 663. Please consult these summaries for a rapid review of the essential material.


Questions on the topics discussed in this chapter can be found in the Clinical Bacteriology section of PART XIII: USMLE (National Board) Practice Questions starting on page 693. Also see PART XIV: USMLE (National Board) Practice Examination starting on page 731.

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