Skip to Main Content

We have a new app!

Take the Access library with you wherever you go—easy access to books, videos, images, podcasts, personalized features, and more.

Download the Access App here: iOS and Android. Learn more here!


Actinomyces and Nocardia are gram-positive rods characterized by filamentous, tree-like branching growth, which has caused them to be confused with fungi in the past. They are opportunists that can sometimes produce indolent, slowly progressive diseases. A related genus, Streptomyces, is of medical importance as a producer of many antibiotics, but it rarely causes infections. Important differential features of these groups and of the mycobacteria to which they are related are shown in Table 28–1.

TABLE 28–1abcdFeatures of Actinomycetes



Actinomycosis is a chronic inflammatory condition originating in the tissues adjacent to mucosal surfaces caused by anaerobic gram-positive branching bacilli of the genus Actinomyces that are present in the microbiota of the alimentary tract. Disease occurs when minor trauma displaces these bacteria below the mucosal barrier. The lesions follow a slow burrowing course with considerable induration and draining sinuses, eventually opening through the skin. The exact nature depends on the organs and structures involved.


Actinomyces are typically elongated gram-positive rods that branch at acute angles (Figure 28–1). They are gram-positive bacilli that grow slowly (4-10 days) under microaerophilic or strictly anaerobic conditions. In pus and tissues, the most characteristic form is the sulfur granule (Figure 28–2). This yellow-orange granule, named for its gross resemblance to a grain of sulfur, is a microcolony of intertwined branching Actinomyces filaments solidified with elements of tissue exudate.

✺ Slow-growing anaerobic branching gram-positive rods

FIGURE 28–1.

Actinomyces. Note the angular branching of the gram-positive bacilli. (Reproduced with permission from Willey JM: Prescott, Harley, & Klein's Microbiology, 7th edition. McGraw-Hill, 2008.)

FIGURE 28–2.

Sulfur granule. The mass is a microcolony of gram-positive bacteria and tissue elements. The branching is clearly seen only at the edge. (Reproduced with permission from Connor DH, Chandler FW, Schwartz DQ, et al: Pathology of Infectious Diseases. Stamford CT: Appleton & Lange, 1997.)

Species of Actinomyces are distinguished on the basis of biochemical reactions, cultural features, and cell wall composition. Most human actinomycosis is caused by Actinomyces israelii...

Pop-up div Successfully Displayed

This div only appears when the trigger link is hovered over. Otherwise it is hidden from view.