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Baron S, editor. Medical Microbiology. 4th edition. Galveston (TX): University of Texas Medical Branch at Galveston; 1996.

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Medical Microbiology. 4th edition.

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Chapter 34Actinomyces, Propionibacterium propionicus, and Streptomyces


General Concepts

Actinomyces and Propionibacterium

Clinical Manifestations

Typical actinomycosis in humans is a chronic disease caused by Actinomyces israelii, A gerencseriae and Propionibacterium propionicus (previously Arachnia propionica). The infection is characterized by persisting swelling, suppuration, and formation of abscesses with draining sinuses. Major types are cervicofacial, thoracic, and abdominal. In addition to the pathogens in actinomycosis some Actinomyces species have also been isolated from other mixed anaerobic infections, eye infections, blood and the urinary tract.


Gram-positive rods grow as filaments, branching rods, and diphtheroidal rods.

Classification, Antigenic and DNA Types

There are 19 species of Actinomyces associated with humans and animals. Most species represent a distinct serologic group, often with subtypes. P propionicus has two serotypes. Recently, strains of some species have been typed by DNA fingerprinting and ribotyping. Classification is based on analysis of DNA and rRNA, cell wall structure, metabolic end products, biochemical reactions, and serology.


In actinomycosis endogenous oral bacteria are introduced into tissue. Abscesses with fibrous walls and pus with sulfur granules develop. Lesions spread by direct extension. A similar etiology may apply to other types of infection, including those with the more recently described Actinomyces, although this is not known with certainty.

Host Defenses

An intact mucosa is the first line of defense. Another possible defense is cell-mediated and humoral immunity.


Actinomycosis is endemic; sporadic cases occur worldwide. Generally the infection is unrelated to age, sex, season, or occupation. Some Actinomyces spp are associated with other mixed anaerobic infections and in infections in severely compromised patients. These can be significant opportunist pathogens.


Actinomycosis is suggested by a suppurative lesion with Gram-positive filaments in the exudate. Sulfur granules may be present. The diagnosis of actinomycosis and other infections involving Actinomyces, including those in compromised patients is confirmed by isolation and identification of the bacteria.


Good oral hygiene may help in prevention of actinomycosis. Actinomycosis is treated with antibiotics and surgical drainage of lesions. In all Actinomyces infections penicillin is the drug of choice.


Clinical Manifestations

Symptoms of mycetoma include actinomycotic mycetoma. Lesions are swollen and localized at the site of the trauma. There are multiple abscesses, draining sinuses, and pus with granules.


Gram-positive bacteria form filaments and branching filaments, a nonfragmenting substrate mycelium, and an aerial mycelium with spores.

Classification and Antigenic Types

Over 400 species have been described based on morphology, pigmentation, cell wall type, lipid composition, and numerical taxonomy. As a result of numerical taxonomy using well-defined stable characters for discrimination, it seems likely that this number of species will be reduced.


Most species are not pathogenic. Streptomyces somaliensis causes mycetoma. Some species cause plant diseases.

Host Defenses

Resistance depends on a well-developed cell-mediated immune system.


Streptomyces species are found worldwide in soil. They are important in soil ecology and as antibiotic producers. Mycetoma occurs mainly in tropical and subtropical areas.


The diagnosis is suggested by clinical features of mycetoma and characteristic pus granules; it is confirmed by isolation and identification of microorganisms.


Treatment is with long-term antibiotics and surgery.


Actinomyces spp and Propionibacterium propionicus (previously Arachnia propionica) are members of a large group of pleomorphic Gram-positive bacteria, many of which fhave some tendency toward mycelial growth. Both are members of the oral flora of humans or animals. Actinomyces species, in particular, are major components of dental plaque. A israelii, A gerencseriae (previously A israelii serotype II), and P propionicus cause actinomycosis in humans and animals. Other species of Actinomyces can be involved in mixed anaerobic and other infections, where they may not always play an obviously pathogenic role. In addition, some coryneform bacteria (diphtheroids) isolated from clinical samples, which had been placed into the Centers for Disease Control Coryneform groups 1, 2 and E, have been identified as new species of Actinomyces.


Clinical Manifestations

Actinomycosis is a chronic disease characterized by the production of suppurative abscesses or granulomas that eventually develop draining sinuses (Fig. 34-1). These lesions discharge pus containing the organisms (Fig. 34-2). In long-standing cases, the organisms are found in firm, yellowish granules called sulfur granules (Fig 34-3). The disease is usually divided into three major clinical types, cervicofacial, thoracic, and abdominal, but primary infections may involve almost any organ. Secondary spread of the disease is by direct extension of an existing lesion without regard to anatomic barriers. Hematogenous secondary spread of the organisms, except from thoracic lesions, is not common.

Figure 34-1. Pathogenesis and disease sites of three major forms of actinomycosis.

Figure 34-1

Pathogenesis and disease sites of three major forms of actinomycosis.

Figure 34-2. Gram stain of A israelii showing diphtheroidal rods and short branching filaments.

Figure 34-2

Gram stain of A israelii showing diphtheroidal rods and short branching filaments.

Figure 34-3. Sulfur granule from human actinomycosis tissue section (hematoxylin and eosin stain).

Figure 34-3

Sulfur granule from human actinomycosis tissue section (hematoxylin and eosin stain). (From Slack JM, Gerencser MA: Actinomyces, Filamentous Bacteria: Biology and Pathogenicity. p. 95. Burgess, Minneapolis, 1975, with permission.)

Actinomycosis is almost always a mixed infection; a variety of other oral bacteria can be found in the lesion with Actinomyces or P propionicus. The role of these associated bacteria is not clear. It has been shown that succinic acid produced by Actinomyces can promote the growth of some Gram -ve anaerobes. Also, in experimental infections, Eikenella corrodens and Actinobacillus actinomycetemcomitans enhance the virulence of Actinomyces. The mycelial masses of Actinomyces reduce the rate of penetration of antibiotics and may physically protect the associated bacteria. In addition, associated organisms which produce ß-lactamases can complicate treatment. Therefore, the dense granules of Actinomyces and the presence of associated bacteria can enhance the virulence of the infection and influence the mode of use of antibiotics, thereby adding to the difficulty of treating the disease.

Cervicofacial infections involve the face, neck, jaw, or tongue and usually occur following an injury to the mouth or jaw or a dental manipulation such as extraction. The disease begins with pain and firm swelling along the jaw and slowly progresses until draining sinuses are produced.

Thoracic actinomycosis results from aspiration of pieces of infectious material from the teeth and may involve the chest wall, the lungs, or both. The symptoms are similar to those of other chronic pulmonary diseases, and the disease is often difficult to diagnose. Thoracic disease may spread extensively to adjacent tissues or organs and often disseminates through the bloodstream, resulting in abscesses in distant sites such as the brain.

Abdominal actinomycosis is often associated with abdominal surgery, accidental trauma, or acute perforative gastrointestinal disease. Persistent purulent drainage after surgery or abdominal masses resembling tumors may be the first sign of infection.

Actinomycosis may affect almost any organ. For example, Actinomyces and P propionicus cause a lacrimal canaliculitis with concretions in the canaliculi that are persistent. A naeslundi i and A odontolyticus can also infect the eyes. In recent years, Actinomyces and P propionicus have been isolated with increasing frequency from female pelvic infections associated with wearing an intrauterine contraceptive device. There has been considerable interest in the possible role of Actinomyces in human periodontal disease and root surface caries. Evidence suggests that these bacteria are not involved in the more destructive forms of periodontal disease. Actinomyces naeslundii and possibly other species may be involved in gingivitis and mild forms of periodontitis. They may also facilitate colonization of the gingivae through coaggregation with Gram -ve anaerobes. Actinomyces naeslundii has also been associated with root surface caries and can be the predominant organism in some lesions.


Despite their name, which means “ray fungus,” Actinomyces are typical bacteria. Both Actinomyces and P propionicus are Gram-positive filamentous rods that are not acid fast and are nonmotile (Fig. 34-4A). As in other Gram-positive bacteria, the cell wall peptidoglycan contains muramic acid, N-acetylglucosamine, glutamic acid, and one or two additional amino acids. Actinomyces species have lysine or lysine plus ornithine in the peptidoglycan, whereas P propionicus contains L-diaminopimelic acid and glycine. Most strains of A viscosus and A naeslundii bear well-developed long, thin surface fibrils. Pili (fimbriae) on A viscosus and A naeslundii are of two types. Type 1 pili are involved in attachment of the bacteria to hard surfaces in the mouth, whereas type 2 pili are involved in coaggregation reactions with other bacteria.

Figure 34-4. Characteristic colonies of A israelii.

Figure 34-4

Characteristic colonies of A israelii. (A) Microcolony at 24 hrs. Brain heart infusion agar shows branching filaments with no distinct center: spider colony. (B) Mature colony at 14 days. Brian heart infusion agar, heaped colony with central depression. (more...)

Classification and Antigenic Types

Actinomyces and P propionicum are irregular, nonspore-forming, Gram-positive rods. Actinomyces contains 19 species (Table 34-1), of which A israelii and A gerencseriae are the most common human pathogens. The genera Actinomyces and Propionibacterium are defined by chemotaxonomic tests, e.g., cell wall composition, fermentation products, cellular fatty acids; DNA homology; and analysis of their ribosomal RNA (rRNA). Using these methods some of the CDC Cornyeform groups (see above) have been classified as Actinomyces.

Table 34-1. Species of Actinomyces.

Table 34-1

Species of Actinomyces.

Actinomyces and P propionicus grow well on most rich culture media. They are best described as aerotolerant anaerobes. The species vary in oxygen requirements: A viscosus and A naeslundii for example, grow best in an aerobic environment with carbon dioxide, whereas A israelii requires anaerobic conditions for growth. Both Actinomyces and P propionicus obtain energy from the fermentation of carbohydrates. The major end products of glucose fermentation by Actinomyces are acetic, lactic, formic, and succinic acids, whereas Propionibacterium produces propionic, acetic, and formic acids with a trace of succinic acid.

All the Actinomyces species that have been examined serologically can be separated from the other species and from P propionicus. A naeslundii, A viscosus, A odontolyticus, and A bovis each have at least two serotypes. P propionicum strains can also be separated into two serotypes.

The chemical composition and cellular location of some Actinomyces antigens are known. One group of carbohydrate antigens is cell-wall associated, protease resistant, and heat stable. Actinomyces viscosus also has an amphipathic antigen that is a fatty acid-substituted heteropolysaccharide and different from the teichoic and lipoteichoic acids found in most Gram-positive bacteria. The pili of A viscosus and A naeslundii are of two antigenic types, which correlate with the different functions of type 1 and 2 pili (see above).

Recently, Actinomyces strains have been characterized on the basis of their DNA fingerprints in attempts to show the transmission of specific oral strains among persons. Also, strains have been ribotyped, by identifying rRNA gene sequences in the DNA fingerprint. Ribotyping of oral A naeslundi i strains has shown that there are up to 5 ribotypes in an individual and that it is uncommon for individuals to share ribotypes. These newer genetic typing methods are likely to improve our knowledge of the biology of Actinomyces, their epidemiology and their role in disease.


Actinomycosis results when bacteria resident in the mouth are introduced into the tissues. The mechanisms by which Actinomyces and P propionicum produce disease are not clear. Pathogenesis may involve the ability of these organisms to suppress some of the immune functions of the host. Studies of oral disease have shown that Actinomyces are chemotactic, activate lymphocyte blastogenesis, and stimulate the release of lysosomal enzymes from polymorphonuclear leukocytes and macrophages.

With the exception of A pyogenes, which produces a soluble toxin and a hemolysin that can be neutralized by antiserum, Actinomyces and P propionicus do not produce exotoxins or significant amounts of other toxic substances. Factors that would aid in tissue invasion and abscess formation have not been demonstrated.

Host Defenses

Antibodies to Actinomyces circulate in some healthy individuals and in individuals with gingivitis and periodontitis, as well as in those with clinical actinomycosis. This humoral response probably does not play a major role in defense against actinomycosis. An intact mucosa is the first line of defense, because Actinomyces and P propionicus, like other anaerobes in the normal flora, must gain access to tissue with an impaired blood supply to establish an infection. Once the organisms have gained access to tissues, the cell-mediated immune response of the host may limit the extent of the infection, but may also contribute to tissue damage. Humoral responses could play a role in infections with some of the other Actinomyces sp, such as A bernardiae and A neuii, which have been isolated from blood cultures.


Actinomyces israelii, A gerencseriae, A georgiae, A naeslundii, A odontolyticus, A meyeri, possibly A pyogenes and P propionicus are normal inhabitants of the human mouth and are found in saliva, on the tongue, in gingival crevice debris, and frequently in tonsils in the absence of clinical disease. Some data suggest that A israelii may also be a common inhabitant of the female genital tract. The natural habitats are not known for the more recently described Actinomyces from human infections A bernardiae, A neuii, A radingae, and A turicensis are not known. Actinomyces bovis is not found in humans and, to date, the other species described from animals, with the exception of A pyogenes, have not been found in human specimens. Among the animal pathogens, A bovis causes lumpy jaw in cattle; A viscosus and A hordeovulneris infect dogs; A pyogenes causes infections in a range of domestic animals, including cattle, sheep and goats; and A suis and A hyovaginalis produce infections in swine. Other species appear to be relatively non-pathogenic, A denticolens, A howellii and A slackii in cattle; A viscosus in rodents and both A naeslundii and A viscosus in zoo animals, including primates. Actinomyces and P propionicus are not found in the soil or on vegetation.

Actinomycosis occurs worldwide. No relationship to race, age, or occupation has been noted, but the disease appears more often in men than in women. Except for human bite wounds, no evidence exists to support person-to-person or animal-to-human transmission of Actinomyces. Compromised patients may be infected by the more recently described Actinomyces.


A search for Actinomyces sp. and P propionicucu s usually is based on a tentative clinical diagnosis of actinomycosis, but these bacteria should be considered whenever a direct Gram stain of pus or suppurative exudate shows Gram-positive, non-acid-fast rods in diphtheroidal arrangements with or without branching. Specimens are first examined for the presence of granules. If present, the granules are crushed, Gram stained, and examined for Gram-positive rods or branching filaments. Washed, crushed granules or well-mixed pus in the absence of granules is cultured on a rich medium, such as brain heart infusion blood agar and incubated anaerobically and aerobically with added carbon dioxide. Plates are examined after 24 hours and after 5 to 7 days for the characteristic colonies of A israelii, A gerencseriae and P propionicus, which resemble those of A israelii (Fig. 34-4B). Colonies of other Actinomyces spp. can take a variety of forms, including smooth domed or flat colonies of various sizes. Isolates morphologically resembling Actinomyces and P propionicus are identified by determining the metabolic end products by gas-liquid chromatography and by performing a series of biochemical tests. Immunofluorescence tests are useful for serologic identification of isolates and the direct demonstration of the organisms in clinical samples.


Due to the mixed nature of the infection and the presence of granules (see above) successful treatment of actinomycosis requires long-term antibiotic therapy combined with surgical drainage of the lesions and excision of damaged tissue. Actinomyces spp and P propionicus are susceptible to penicillins, the cephalosporins, tetracycline, chloramphenicol, and a variety of other antibiotics. Penicillin is the drug of choice for infections with all species of Actinomyces; significant drug resistance is unknown.


Like Actinomyces and Propionibacterium, Streptomyces belongs to the large group of filamentous bacteria known as actinomycetes, but Streptomyces species have a well-developed substrate mycelium, produce an aerial mycelium with chains of spores, and are strict aerobes. Streptomyces are common in soil and give it its characteristic earthy odor. They seldom produce human infections, but are important as producers of antibiotics.

Clinical Manifestations

Some Streptomyces species, principally S somaliensis, cause actinomycotic mycetoma, which is indistinguishable from that caused by Nocardia species (see Ch. 33), other actinomycetes, and some fungi. Lesions usually occur on the extremities, most often on the feet (Fig. 34-5). They appear as localized swollen nodules that slowly enlarge. Multiple abscesses form, and draining sinuses open to the surface and discharge pus and granules.

Figure 34-5. Pathogenesis of actinomycotic mycetoma.

Figure 34-5

Pathogenesis of actinomycotic mycetoma.


Streptomyces species are Gram-positive, aerobic, filamentous bacteria with an extensive substrate mycelium that does not fragment and aerial hyphae with chains of spores produced by hyphal segmentation. Colonies are initially smooth but become powdery or cottony as the aerial mycelium and spores develop. The colonies grow slowly, requiring 7 to 10 days to develop aerial hyphae.

Classification and Antigenic Types

Species identification within the genus Streptomyces is complicated. More than 400 species have been described on the basis of morphology, pigmentation, cell wall structure, chemical composition, some biochemical tests, and serologic relationships. Streptomyces species have been divided into seven groups on the basis of the color of the mature aerial mycelium. Further subdivision is based on the morphology of the spore chains. In addition, cell wall peptidoglycan structure and the types of sugars in whole-cell hydrolysates are used extensively to characterize aerobic actinomycetes. Streptomyces has a type 1 cell wall containing L-diaminopimelic acid and glycine and no characteristic cell wall sugar. Chemical analyses and numerical taxonomy have identified 77 groups (clusters) of Streptomyces, some of which comprise a single species, while others include several species (species groups). Although the results of these studies suggest that the numbers of species of Streptomyces should be reduced, the genus will remain complex.


Soil organisms are introduced into the tissue by trauma, most often minor trauma caused by thorns, splinters, or an abrasion (Fig. 34-5). The initial lesions spread to subcutaneous tissue and then into bone. Radiographs show multiple small granulomas with pus in the center and cavities in the bone. Continued spread of the cutaneous lesions leads to the formation of draining sinuses. The discharged pus contains granules, whose size and color provide a clue to the etiologic agent. For example, S somaliensis granules are large and yellow to brown.

Host Defenses

Mycetoma develops in only some of the many individuals exposed to the organisms. Development of disease seems to be dependent on a deficiency in cell-mediated immunity. A relationship between susceptibility to mycetoma and deficiency in cell-mediated immunity has been shown in animal studies.


Streptomyces spp occur worldwide in the soil, in water, and on organic debris. They play an important role in soil ecology by decomposing organic matter and contributing to soil fertility. The major medical significance of this genus is as a producer of antibiotics. About 85 percent of the known antibiotics, including streptomycin, chloramphenicol, and tetracycline, are produced by streptomycetes.

Mycetoma occurs mainly in tropical and subtropical areas. The dominant agent in a particular area depends on the prevalence of these agents in the soil.


Diagnosis is usually based on clinical grounds when there is a well-established lesion. Determination of antibodies in serum, usually by counter-immunoelectrophoresis, is useful. Pus should be examined for the color, size, consistency, and microscopic appearance of granules, which will usually identify the agent of mycetoma. Definitive diagnosis depends on isolation and identification of the bacteria.


Long-term antibiotic treatment and surgical management are necessary. In vitro, S somaliensis is sensitive to rifampicin, erythromycin, tobramycin, fusidic acid and streptomycin. Strains tested were resistant to trimethoprim. For S somaliensis infection, treatment with streptomycin and either co-trimoxazole or dapsone is recommended. The average duration of treatment is about 10 months.


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Copyright © 1996, The University of Texas Medical Branch at Galveston.
Bookshelf ID: NBK8385PMID: 21413327


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