Chapter 22: Actinomycetes

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).

Additional information regarding the clinical aspects of infections caused by the organisms in this chapter is provided in Part IX entitled Infectious Diseases beginning on page 607.

Disease

Actinomyces 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.

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. Note that in contrast to N. asteroides (see later), Actinomyces is not acid-fast. 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.

Disease

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 to decolorize than that used in the stain for mycobacteria). If the regular-strength acid is used, N. asteroides will decolorize. N. asteroides is weakly acid-fast because it contains a layer of mycolic acid in its cell wall, similar to, but less than, the mycobacteria.

Clinical Findings

N. asteroides typically causes 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 (see 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.