Actinomycetes Overview

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Agar plate culture of Actinomyces sp. bacteria, a Gram positive opportunistic pathogen of the oral cavity. (Marco Tollo 2006, Wikimedia commons)

The actinomycetes comprise a group of procaryotes that have the ability to form Gram-positive, branching filaments of less than 1μm in diameter. The main animal pathogens in the actinomycetes are the genera Actinomyces, Nocardia and Dermatophilus.

Streptomyces species are also included in this group, and they are prolific producers of antimicrobial substances.

Actinomyces species

Morphology and Staining

These are gram-positive diphtheroid or filamentous rods, about 0.5μm in width. Branching filaments are usually found in pathological specimens. In culture, diphtheroid forms predominate.

Structure and Composition

Actinomyces species have distinctive cell wall constituents. Surface fibrils in A. viscosus may be adhesins for host cells or other bacteria. Surface antigens are related to chemotactic and mitogenic activities.

Growth Characteristics

Animal Actinomyces species are capnophilic or facultative anaerobes. All require rich media, preferably containing serum or blood. No growth occurs on Sabouraud's agar. Development of macroscopic colonies may require several days at 37°C. Colonical morphology varies between and within species.

Biochemical Activities

Actinomyces are catalase-negative, with the exceptions of A. viscosus and A. canis.


Actinomyces species live in the nasopharyngeal and oral mucosa, and secondarily in the gastrointestinal tract.


Actinomyces species evoke pyogranulomatous reactions by unknown mechanisms. Bacterial colonies trigger the suppurative response, and there is also granulation, mononuclear infiltration and fibrosis.

Sinus tracts carry the exudate to the outside. This often contains 'sulfur granules' which are bacterial colonies surrounded by clubs.

Diseases caused by Actinomyces species include:

Ruminants: A. bovis causes lumpy jaw.
Horses: Actinomyces species have been isolated from cases of Poll Evil and Fistulous Withers as well as in cases of cervical lymphadenitis.
Dogs and cats: Actinomyces are common causes of pyonecrotic processes, usually associated with a foreign body such as a migrating grass awn. Commonly isolated species include: Actinomyces viscosus, Actinomyces hordeovulneris, A. canis, A. catuli.
Pigs: mastitis, pneumonia and abortion have all be associated with Actinomyces infections.

Nocardia species

Morphology and Staining

Gram-stained Nocardia species are indistinguishable from Actinomyces species. They are partially acid-fast so a Ziehl-Neelsen stain would differentiate them from Actinomyces. They alternate between the coccobacillary resting phase and the actively-growing filamentous forms.

Structure and Composition

The cell wall is typical of gram-positive bacteria. It contains a high concentration of lipids. A superoxide dismutase acts as a virulence factor.

Growth Characteristics

Nocardia species are obligate aerobes growing on simple media (Sabouraud's) over a wide temperature range. Colonies are opaque and variously pigmented. The colony surface, which is waxy to powdery to velvety depending on the abundance of growth, wrinkles with age. The diameter of colonies may reach several centimetres.

Biochemical activities include catalase production and acidification of various carbohydrates.


Pathogenic Nocardia species are saprophytic and found in many climates in soil and water, either as indigenous flora or as contaminants.


Nocardia species survive within phagocytic vacuoles and lead to a predominantly suppurative process. Lymph nodes are consistently involved. Exudates are serosanguinous and sometimes contain 'sulfur-granule-like' debris, which lack the microstructure of Actinomyces sulfur granules.

Diseases in different species include:

Ruminants: mastitis, bovine farcy, pneumonia, abortion and lymphadenitis
Horses: local or general infections due to underlying immunosuppression (EPPID), placentitis which causes abortion
Dogs and cats: pneumonia, suppurative pleuritis with empyema, dissemination to kidneys, CNS, bones, joints
Pigs: pneumonia, abortion, lymphadenitis

Dermatophilus species

Dermatophilus congolensis is the only species, and is a gram-positive filamentous bacterium.

Morphology and Composition

The reproductive unit of the bacterium is the motile coccoid zoospore. Upon germinating, the zoospores sprout a germ tube which elongates and thickens, dividing transversely and longitudinally, forming a strand several layers thick. Multiflagellated zoospores are liberated as the strand disintegrates.

Growth Characteristics

D. congolensis grows on blood media, but not on Sabouraud's agar. It is aerobic and capnophilic. Haemolytic colonies develop within 48 hours and vary from mucoid to viscous and waxy and whitish-grey to yellow. Catalase, urease and proteases are produced. The bacterium survives well in the soil and on fomites.


D. congolensis does not multiply saprophytically. Its reservoir is infected animals. Cattle, sheep, goats and horses are common hosts but it has been diagnosed in many other animals.


It causes an exudative dermatitis, with activity confined to the living epidermis. There is a scab consisting of layers of neutrophilic exudate and keratinising epidermis. Wetting favours expansion of the lesions.

It causes lesions on the back (rain scald in horses, lumpy wool in sheep), feet and legs (strawberry footrot in sheep and grease heel in horses).

Streptomyces species

These are not pathogenic but are similar to Nocardia species in cultures. Some species produce antibiotics, and they are common laboratory contaminants.

Actinomycetes Overview Learning Resources
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Quinn, P. (1994) Clinical veterinary microbiology Elsevier Health Sciences

Hirsh, D. (2004) Veterinary microbiology Wiley-Blackwell

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