Actinomycetes are true bacteria (related to corynebacteria and mycobacteria), but they form long, branching filaments that resemble the hyphae of fungi.
They are gram-positive, but some (such as Nocardia asteroides) are also weakly
There are two medically important organisms:
; Actinomyces israelii
; N. asteroides.
Color Plate 12
Nocardia asteroides—Gram stain. Arrow points to area of filaments of gram-positive rods. Provider: Professor Shirley Lowe, University of California, San Francisco School of Medicine. With permission.
Table 22–1. Actinomycetes.
Species Disease Habitat Growth Diagnosis Treatment
A. israelii Actinomycosis Oral cavity Strictly Gram-Penicillin G
(abscess with anaerobic positive
N. Nocardiosis Environment Aerobic Gram-Sulfonamides
asteroides (abscesses in positive
brain and branching
kidneys in filaments;
immunodeficient often acid-
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. Hard, yellow granules (sulfur granules) composed of a mass of filaments are formed in pus.
Actinomycosis appears as a hard, nontender swelling that develops slowly and eventually drains pus through sinus tracts. 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.
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.
Nocardia 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.
N. asteroides and Nocardia brasiliensis are the most common causes of human
nocardiosis. The disease begins as a pulmonary infection and may progress to form abscesses and sinus tracts. Unlike A. israelii, sulfur granules are not formed.
In immunocompromised persons, the organism may spread to the brain, skin, or kidneys. The disease is not communicable.
Diagnosis in the laboratory involves
(1) Seeing branching rods or filaments that are gram-positive or weakly acid-fast (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.