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Actinomycetes
Last reviewed: 06.07.2025
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Actinomycetes are branching bacteria. They do not contain chitin or cellulose in the cell wall, unlike fungi, and have the structure of gram-positive bacteria. The mycelium is primitive. Thin straight or slightly curved rods measuring 0.2-1.0x2.5 μm, often forming threads up to 10-50 μm long.
Capable of forming a well-developed mycelium, in some species it is long, rarely branching, in others it is short and strongly branching, the hyphae of the mycelium are not septate. Rod-shaped forms, often with angular ends, are located in the smear singly, in pairs. V- and Y-shaped or in the form of a palisade. All morphological forms are capable of true branching, especially on a thioglycollate semi-liquid medium. They stain poorly according to Gram, often form granular or bead-like forms, do not form conidia, are not acid-resistant. The typical species is Actinomyces hovis.
Cultural properties of actinomycetes
Obligate and facultative anaerobes, capnophiles. Grow slowly, crops should be cultivated for 7-14 days. Temperature optimum for growth is 37 C. Some strains cause hemolysis on media with blood. Some species form filamentous microcolonies resembling mycelium, and on the 7-14th day they form crumbly S-shaped colonies, sometimes colored yellow or red. Actinomyces israelii tends to form a long branching mycelium, which eventually disintegrates into polymorphic coccoid, tubular and other elements. It grows poorly on simple nutrient media, grows better on protein media containing serum; forms transparent, colorless, pasty, usually smooth colonies that tightly grow together with the medium. The aerial mycelium is sparse, does not form pigments, on some media, for example on blood agar, it can form white bumpy colonies, A. odontoiyticm on blood agar forms red colonies with a zone of hemolysis.
Biochemical activity of actinomycetes
Actinomycetes are chemoorganotrophs. They ferment carbohydrates to form acid without gas; the fermentation products are acetic, formic, lactic and succinic acids (but not iropionic). The presence of catalase and the ability to reduce nitrates to nitrites vary among species; they do not form indole. Species differentiation is based on differences in the ability to ferment carbohydrates and in some other biochemical tests.
In ELISA, serogroups A, B, C, D, E, F are distinguished.
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Ecological niche of actinomycetes
The main habitat is soil. They are constantly found in water, air, on various objects, plant, animal and human skins. They colonize the oral mucosa of humans and mammals.
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Persistence of actinomycetes in the environment
They die instantly when exposed to air.
Sensitivity to antimicrobial drugs. Sensitive to penicillins, tetracycline, erythromycin and clindamycin. Sensitive to the action of commonly used antiseptics and disinfectants.
Epidemiology of actinomycosis
The source of infection is soil. It is characterized by a multiplicity of mechanisms, routes and factors of transmission, although the purest mechanism of transmission is contact, and the route of transmission is wounded. Susceptibility to actins and zetas, as to all opportunistic microbes, is low in individuals with normal immune status and increased in immunodeficient hosts.
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Symptoms of actinomycosis
Actinomycosis is a chronic opportunistic infection of humans and animals caused by anaerobic and facultative anaerobic actinomycetes, which is characterized by granulomatous inflammation with polymorphic symptoms.
Actinomycosis disease is manifested by the formation of a granuloma, which undergoes necrotic decay with the formation of pus, which comes out through fistulas on the surface of the skin and mucous membranes of various consistencies, yellowish-white in color, sometimes with an admixture of blood, often contains druses. At the same time, fibrosis of the granuloma is noted. Depending on the localization, cervicofacial, thoracic, abdominal, genitourinary, osteoarticular, skin-muscular, septic and other forms of the disease are distinguished.
Immunity has not been sufficiently studied.
Laboratory diagnostics of actinomycosis
The material for the study includes sputum, cerebrospinal fluid, pus from fistulas, punctures from unopened foci of softening, scrapings from granulations, and tissue obtained during biopsy.
To diagnose actinomycosis, bacterioscopic, bacteriological, serological and allergological methods are used.
Usually the diagnosis is made bacterioscopically by detecting actinomycete druses in the material being examined, which look like small yellowish or grayish-white grains with a greenish tint. Under low magnification, formations of a circular shape with a structureless center and a radial periphery are visible; under high magnification, plexuses with pigmented grains are visible in the center, and hyphae with flask-shaped thickenings at the ends extend radially from this mycelial ball along the periphery. According to Gram, the spores are stained dark purple, the mycelium is purple. According to Ziehl-Nelson, the mycelium is stained blue, and the spores are red.
The final diagnosis is established based on the isolation of the pathogen. To suppress the growth of accompanying microflora, pus and sputum are centrifuged in a solution of penicillin and streptomycin before sowing, then washed with an isotonic solution of NaCI to remove antibiotics. They are inoculated onto nutrient media (sugar agar, Sabouraud medium, etc.) and cultivated under aerobic and anaerobic conditions. A pure culture is isolated and identified according to the generally accepted scheme. The ability to coagulate and peptonize milk is determined in the isolated cultures - a feature characteristic of actinomycetes. Isolation of anaerobic species confirms the diagnosis of actinomycosis.
For serodiagnosis of actinomycosis, a complete blood count (CBC) with actinolysate is used. The reaction is not specific enough, since positive results can be observed in lung cancer and severe suppurative processes. The use of extracellular actinomycete proteins as an antigen instead of actinolysate increases the sensitivity of the CBC. The same antigen can be used to perform the RNGA.
An allergic test is performed with actinolysate. Only positive and strongly positive tests have diagnostic value. With visceral actin and goat, the allergic test is often negative.