Morphological Features and Structure
The name of this group of microorganisms comes from the Greek roots actis (ray) and mykes (fungus). Despite their superficial resemblance to eukaryotic fungi, actinomycetes are true bacteria. In smears, they can appear singly, in pairs (forming V- or Y-shaped figures), or in characteristic palisade arrangements.
A key feature of members 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 nutrient medium.
- Aerial mycelium — develops above the surface of the medium.
Reproduction occurs via fragmentation of the mycelial threads into individual rod-shaped or coccoid segments, as well as through the formation of exogenous spores. Exospores are formed exclusively on aerial hyphae. Their primary function is reproduction; unlike the endospores characteristic of bacilli, actinomycete spores possess zero heat resistance.
Cultural and Biochemical Properties
The causative agents of actinomycosis are highly demanding regarding cultivation conditions. Most of them are obligate or facultative anaerobes, as well as capnophiles (requiring 6–10% CO₂). The optimal growth temperature is 35–37 °C.
Rich nutrient bases are used for cultivation, such as thioglycolrate medium or brain-heart infusion broth with the mandatory addition of hemin and vitamin K₃. Growth is slow: the first signs appear no earlier than 48 hours, and full colonies grow within 7–14 days. They firmly adhere to the agar. Rough R-forms have a characteristic nodular appearance reminiscent of molars.
Regarding their nutrition type, these bacteria are chemoorganoheterotrophs. They ferment sugars with the production of acids (lactic, acetic, formic, and succinic), but without gas formation. An important diagnostic sign: actinomycetes never synthesize propionic acid or 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 host resistance is weakened, actinomycosis develops—a chronic opportunistic infection. The primary etiologic agent is Actinomyces israelii.
Several forms of the disease are distinguished depending on the portal of entry:
- Cervicofacial (most common) — bacteria invade through dental caries.
- Thoracic (pulmonary) — occurs due to aspiration of oral contents.
- Abdominal — associated with trauma to the intestinal mucosa or appendix.
The pathological process begins with the formation of a localized induration (granuloma) around the pathogen colony. Gradually, the infiltrate softens, multiple coalescing abscesses form, and they rupture through the skin as fistulous tracts. Surrounding tissues undergo fibrosis. The purulent exudate contains specific dense formations 1–2 mm in diameter—"sulfur" granules (Bollinger bodies), consisting of microbial clusters (druse). The development of suppuration is frequently associated with secondary flora (staphylococci) and allergic alteration of the immune system.
Laboratory Diagnostics and Treatment Principles
Specimens include purulent discharge from fistulae, aspirates, sputum, or tissue biopsies. It is critical to transport the material to the laboratory strictly under anaerobic conditions using specialized collection devices.
Diagnostic algorithms include:
- Bacterioscopy. Smears are examined for "sulfur" granules. Under the microscope, a drusen appears as a central tangle of filaments from which hyphae radiate outward with characteristic club-shaped swellings.
- Bacteriological culture. Isolation of a pure culture serves as the "gold standard" for definitive confirmation of the diagnosis, despite the prolonged incubation period.
- Allergic diagnostics. Intradermal skin testing with actinolysate. A positive reaction has diagnostic value, though visceral forms may yield false-negative results.
Therapy is multimodal. Etiotropic treatment involves antibacterial drugs (penicillins, tetracyclines, erythromycin). Additionally, immunotherapy using actinolysate is administered. Specific vaccines for the prevention of actinomycosis have not been developed.