Chapter 28

Actinomycetes & Mycobacteria

S. G. Gbadamosi




28.1.1 General characteristics

  • 1. Bacteria that tend to form branching filaments which in
  • some families develop into a mycelium;
  • 2. Some families produce true spores;
  • 3. They are gram positive, though this reaction may vary with
  • the age of the culture;
  • 4. The mycobacteria are acid-alcohol-fast and some members
  • of the Nocardiaceae are weakly acid-fast;
  • 5. All members of the order are aerobic except for the genera in
  • the Actinomycetaceae, which may be anaerobic, facultatively
  • anaerobic or aerobic.

In this section, only four of the eight known families will be considered in that they contain the genera know to have pathogenic significance in man and animals.

28.1.2 Classification:


Family: Actinomycetaceae

Genus: Actinomyces

Family: Mycobacteriaceae

Genus: Mycobacterium

Family: Dermatophilaceae

Genus: Dermatophilus

Family: Nocardiceae

Genus: Nocardia

Table 1 contains the important species of each genus listed in the classification above. The list is by no means exhaustive and any unlisted species will be discussed under the appropriate genus. Note that the genus Mycobacterium has a group name in place of the proper species name. This is necessary in view of the large number of species under the category. Also note that the group name is not a taxonomic name to be used as a substitute for species.


_________________________Table 1.__________________________

Species of disease producing genera of the order Actinomycetales


Actinomyces Mycobacterium Dermatophilus Nocardia

A. bovis M. leprae D. congolensis N. farcinica

A. israeli M. tuberculosis N asteroides

A. odontoly- M. bovis N. brasiliensis


A. viscosus M. avium complex N.otitidis


A. naeslundi M.O.T. N. transvalensis

A suis



Note: M.O.T. stands for Mycobacteria other than tuberculosis, which in some texts will be classified as "atypical mycobacteria", an unfortunate name to say the least.


Genus: Actinomyces


Actinomyces bovis is the name used by Harz in 1878 to describe the mold-like organism observed in the lesion of tongue and jaw of cattle studied by Bollinger in 1877. The disease was called actinomycosis. In 1891, Wolff and Israel cultivate A. israeli anaerobically from cases of human actinomycosis. Since then other species have been recovered from disease conditions in humans and domestic animals.

28.1.3 Habitat:

It is generally agreed that members of this genus are parasitic, or at least commensalistic in nature in that they have not been isolated from outside the body of humans and domestic animals. 

They occur in the region of the mouth, pharynx and tonsils of animals and man. Diseases produced by that species of this organism have been reported worldwide.

Cultural characteristics vary between the species of Actinomyces. However, their cultivation does not present problems in the laboratory. Brain heart infusion agar with blood supports the growth of the most important species. 

A. bovis grows as flat, pale yellow colonies in 4 to 7 days. It is nonhemolytic and nonadherent to the culture media. The colonies are about 1mm in diameter. In contrast the colonies of A. israeli are rough and hard and are markedly adherent to the media. Table 2 describes some of the environmental requirements for the cultivation of Actinomyces species.


Table 2. Environmental Requirements of the species of Actinomyces



A. bovis anaerobic to microaerophilic

A. israeli anaerobic

A naeslundi fucultative anaerobe

A. odontolyticus fucultative anaerobe

A. viscosus facultative anaerobe with extra carbon dioxide requirement.


Biochemical characteristics. In general the species of Actinomyces are non-proteolytic organisms. The chromatographic analysis of the products of carbohydrate fermentation shows acetic, formic, lactic and succinic acids but not propionic acids. With the exception of A. viscosus all species are catalse negative.

Epidemiology and Pathogenesis. All animals, including humans are susceptible to infection by the species of Acinomyces resident in the body. The following discusses the diseases produced by host.

 A bovis - produces diseases in cattle as the result of damage to the buccal epithelium and subsequent inoculation of the organism into the site. Thus, the common areas for infection are the cheek muscles, the lower jaw, the submaxillary region, and less frequently other areas of the throat and head; in swine a frequent site of infection is the mammary gland an it may be that this arises from the organism being in the mouths of piglets, becoming inoculated through abrasions of the skin during suckling. The lungs and other internal organs are occasionally affected; in horses, there is evidence of A. bovis as one of the pathogens implicated in lesions of fistulous withers and poll-evil in association with Brucella abortus infections. 

Actinomyces viscosus is the main pathogenic species found in the dogs, though other animals including humans have developed lesions as a result of infection by this organism. It is implicated in periodontal disease of humans and hamsters; in the dogs, A. viscosus is probably more common than was previously recognized. Some authors even contend that some of those diseases diagnosed as nocardiosis were probably actinomycosis. Two forms of actinomycosis have been recognized: the more common is the localized granulomatous abscess of the skin and subcutis, the form that responds well to antimicrobial therapy; while the other form principally involves the thorax, with or without extension to the abdominal cavity. Pyothorax with granulomatous lesions of thoracic tissues and accumulation of pleural and pericardial fluid containing grey-white granules are characteristic of this deep form. It is this deep form of actinomycosis in the dog that is most difficult to treat, radical surgery is often the treatment of choice along with prolonged antibiotic therapy.

NOCARDIACEAE general characteristics:

  • 1. obligate aerobes
  • 2. gram positive
  • 3. some strains are acid-fast
  • 4. some produce true mycelia
  • 5. there are three morphological groups:

    a. Group I strains have limited mycelia development due toearly fragmentation of hyphae into coccoid forms within 2 to 14 hours of incubation;

    b. Group II strains produce mycelia which fragment in about 18 to 20 hours after incubation, though these mycelia break up into mycelial fragments within two days of growth;

    c. The pathogenic Nocardia species belong to Group III. The colonies are usually leathery in appearance and pigmented. Subsequent discussion concentrate on these species. Extensive mycelium produced because fragmentation does not begin until after 5 days incubation.

  • 6. all species are non-motile,
  • 7. all species are soil borne saprophytes.

28.1.7 History.

Nocard described this organism in 1888, following its isolation from a case of bovine 'farcy', hence the name of the type species: Nocardia farcinica.

 28.1.8 Pathogenic species characteristics

  • 1. see 5c above
  • 2. acid-fast to partially acid-fast,
  • 3. substrate mycelium usually white or gray in color, Nocardia farcinica
  • 4. substrate mycelium some shade of yellow, orange, or red,
    • a. nitrates reduced to nitrites.
    • b. xanthine hydrolyzed; Nocardia otitidis-caviarum
    • c. xanthine not hydrolyzed; gelatin hydrolyzed; Nocardia brasiliensis
    • d. gelatin not hydrolyzed; Nocardia asteroids
    • e. nitrates not reduced to nitrites; Nocardia transvalensis

28.1.9 Pathogenicity.

Nocardia species have been isolated form a variety of clinical situations, though the genus is in general an opportunistic pathogen. In primary nocardiosis, severe, suppurative or cavitary pulmonary infection simulating tuberculosis is often observed, and in some cases cutaneous and subcutaneous abscesses which are diagnostic of N. asteroids and N. brasiliensis. Blood stream invasion with secondary, often fatal, involvement of the internal organs and the central nervous system are seen in N. asteroids.

In the bovine species the most important disease condition is mastitis whose presentation is in the form of extensive fibrosis. N. asteroids is the most often isolated species.

In man, the opportunistic nature of the organism is commonly observed in patients with underlying diseases, especially neoplastic, in which the patient is receiving intensive medical therapy. Particularly implicated are steroids and antineoplastic drugs. Other susceptible compromised patients are organ transplant patients, those with chronic pulmonary disease, cirrhosis of the liver and severe wounds.

Actinomycotic mycetoma which is a swollen, indurated skin lesion which eventually involves the bone, with discharge of pus from sinuses, is the infection type seen in the dogs. Cultural isolation and identification of organisms from this lesion will be necessary to differentiate it from that produced by Actinomyces.


  • 1. Infection with Nocardia is by inhalation from the environ-ment, while Actinomyces infection begins in the host as a normalflora invading damaged tissues;
  • 2. Disseminated disease caused by Nocardia is more common in the dogs, while granuloma formation is the rule with Actinomyces infection and spreads by local extension;
  • 3. When there is a doubt as to whether a patient has actinomycosis or nocardiosis, precautionary measures to preserve the anaerobic Actinomyces should be institute, including prompt delivery to the laboratory under anaerobic condition, culturing on brain heart infusion agar blood plates and incubating under anaerobic, microaerophilic, and aerobic conditions. While Actinomyces fails to grow on Sabouraud agar, Nocardia grows uninhibited.
  • 4. Acid-fast staining procedure of the sample exudate before culturing will also be helpful in the presumptive identification of the infecting agent.


28.2.1 General characteristics

  • 1. A group of bacteria with mycelial filaments which divide transversely and in at least two longitudinal planes to form masses of coccoid or cuboidal cells, which characteristically become motile.
  • 2. Aerial mycelium stimulated by 10% CO2,
  • 3. Gram positive,
  • 4. Not acid-fast,
  • 5. Aerobic,
  • 6. Produce pigment, and
  • 7. Includes pathogens causing skin lesions in mammals, including man.

Type genus is Dermatophilus

  • 1. Non-fermentative although acid is produced from certain carbohydrates;
  • 2. Catalase positive;
  • 3. Aerobic to facultatively anaerobic;
  • 4. Pathogenic for mammals, invading only the un cornified epithelium.
  • 5. Optimum temp. 37`C.

Type species: Dermatophilus congolensis

Other species include: Dermatophilus dermatonomus and Dermatophilus pedis

These two species are not described in Bergey's Manual of Determinative Bacteriology, hence they are not acceptable as true but and descriptive species only.

28.2.2 Disease.

The bacterium produces disease in many animal species, it is however, found most commonly in cattle, sheep, goats, and horses. It is also a soonosis. The common name for the disease is dermatophilosis or streptothricosis.

Bovine disease was first described by Van Saceghem in 1915 in the Congo now known as Zaire in Africa.

As far as is known the bacterium is considered an obligate parasite, living only on animals. Infection is spread by contact, biting insects, fomites and by other unknown means. Moist conditions and high relative humidity are known to promote the prevalence of the disease. It has also been speculated that the disease is endemic in cattle. The superficial layers of the skin are almost always involved in the infection, leading to the formation of crusts or scabs varying in size from barely visible lesions to about two inches in diameter. Severe infection leads to extensive coalescence of the smaller lesions.

Though the disease does not lead to death in the adult cattle, deaths in young goats and cattle have been reported in literature.

In sheep, the disease is called mycotic dermatitis and is seen in three forms:

  • 1) dermatitis of the wool-covered areas of the body or lumpy wool;
  • 2) dermatitis of the face and scrotum; and
  • 3) dermatitis of the lower leg and foot, which may result in severe ulcerative dermatitis referred to as "strawberry foot rot".

Infections have been reported in dogs and cats. In dogs, spontaneous dermatophilosis has been confined to the intugementary structures; whereas deeper tissue lesions in the form of abscesses have been reported in the felines. Lymph node enlargement with draining fistulas of the subcutaneous and submucosal tissues have also been reported in cats.

28.2.3 Direct examination.

Smears are made from scabs softened with distilled water and stained with Giemsa or Gram method. Segmenting filaments and coccoid spores stain deep purple. The spores are seen in packets.

28.2.4 Isolation and Cultivation.

The organism grows well on blood agar plates within 24 to 48 hours as small grayish white colonies turning yellow to orange upon further incubation.

Identification of this bacterium does not seem to pose any problems. Lesion and typical morphology are quite diagnostic.

28.2.5 Immunity.

There is no known vaccine for immunization against the disease produced by Dermatophilus. Recovery from the disease seems to confer permanent immunity to reinfection.