Actinomycetes

Morphological features and structure

The name of this group of microorganisms comes from the Greek roots actis (ray) and mykes (fungus). Despite their external similarity to eukaryotic fungi, actinomycetes are true bacteria. In smears, they may appear singly, in pairs (forming V- or Y-shaped figures), as well as in a characteristic palisade arrangement.

A key feature of representatives of the genus Actinomyces is the ability to form a well-developed non-septate mycelium. It is divided into two functional types:

• Substrate mycelium — grows deep into the thickness of the nutrient medium.
• Aerial mycelium — develops above the surface of the medium.

Bacteria reproduce by fragmentation of mycelial filaments into individual rod-shaped or coccoid segments, as well as through the formation of exogenous spores. Exospores are formed exclusively on aerial hyphae. Their main function is reproduction; unlike endospores characteristic of bacilli, actinomycete spores have absolutely no heat resistance.

Cultural and biochemical properties

Actinomycosis causative agents are very demanding regarding cultivation conditions. Most of them are obligate or facultative anaerobes, as well as capnophiles (requiring the presence of 6–10% CO₂). The optimum temperature for reproduction is 35–37 °C.

Rich nutrient bases such as thioglycolate medium or brain-heart infusion broth with the mandatory addition of hemin and vitamin K₃ are used for cultivation. Growth is slow: the first signs appear no earlier than 48 hours later, and full-fledged colonies grow in 7–14 days. They firmly fuse with the agar. Rough R-forms have a characteristic bumpy appearance resembling molars.

According to their type of nutrition, bacteria are chemoorganotrophs. They ferment sugars with the release of acids (lactic, acetic, formic, and succinic), but without gas formation. Important diagnostic sign: actinomycetes never synthesize propionic acid and indole. Their antigenic profile is represented by six serological groups (A–F).

Actinomycosis: clinical presentation and pathogenesis

Actinomycetes are part of the normal microflora of the gastrointestinal tract and oral cavity. When the host organism's resistance is weakened, actinomycosis develops — a chronic opportunistic infection. The main etiological agent is Actinomyces israelii.

Several forms of the disease are distinguished depending on the portal of entry:

1. Cervicofacial (the most frequent) — bacteria penetrate through carious cavities in teeth.
2. Thoracic (pulmonary) — occurs upon aspiration of oral contents.
3. Abdominal — associated with trauma to the intestinal mucosa or appendix.

The pathological process begins with the formation of induration (granuloma) around the pathogen colony. Gradually, the infiltrate softens, multiple coalescent abscesses form, which break through to the skin as fistulas. The surrounding tissues undergo fibrosis. Specific dense formations 1–2 mm in diameter are detected in the purulent exudate — "sulfur" granules (Bollinger bodies), consisting of accumulations of the microbe (drusen). The development of suppuration is often associated with the addition of secondary flora (staphylococci) and an allergic reorganization of the immune system.

Laboratory diagnostics and treatment principles

Purulent discharge from fistulas, aspirates, sputum, or tissue biopsy specimens are taken for analysis. It is extremely crucial to deliver the material to the laboratory strictly under anaerobic conditions using special collectors.

The diagnostic algorithm includes:

• Bacterioscopy. "Sulfur" granules are searched for in smears. Under the microscope, a druse is a central tangle of filaments from which hyphae with characteristic club-shaped thickenings radially depart (in the form of rays).
• Bacteriological culture. Isolation of a pure culture serves as the "gold standard" for the final confirmation of the diagnosis, despite the long cultivation times.
• Allergy diagnostics. Performance of skin tests with actinolysate. A positive reaction is of diagnostic significance, but in visceral forms, the test may yield a false-negative result.

Therapy is complex. Antibacterial drugs (penicillins, tetracyclines, erythromycin) are used as etiotropic treatment. Additionally, immunotherapy using actinolysate is performed. Specific vaccines for the prevention of actinomycosis have not been developed.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Cell wall structural features of nocardioform actinomycetes
• Mechanism of acid-fastness in mycobacteria and Ziehl-Neelsen staining
• Role of secondary flora in the pathogenesis of cervicofacial actinomycosis
• Actinolysate immunotherapy: mechanism of action of the drug
• Differential diagnosis of actinomycosis and nocardiosis

Other topics in the section "Morphology and Classification of Microbes"

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