Introduction
Actinomyces viscosus is a gram-positive, facultative anaerobic, non-motile, and non-spore-forming bacterium that is part of the normal flora of the human oral cavity. It plays a significant role in dental plaque formation and has been implicated in a range of periodontal diseases. Although usually non-pathogenic in healthy individuals, A. viscosus can become opportunistic under certain conditions, leading to oral, respiratory, and even systemic infections.
Taxonomy and Classification
- Domain: Bacteria
- Phylum: Actinobacteria
- Order: Actinomycetales
- Family: Actinomycetaceae
- Genus: Actinomyces
- Species: A. viscosus
Closely related species include Actinomyces naeslundii and Actinomyces israelii, with which it shares morphological and genetic similarities.
Morphology and Physiology
Actinomyces viscosus appears as:
- Gram-positive, filamentous rods or branching bacilli
- Facultative anaerobic, capable of growth with or without oxygen
- Non-motile and non-spore-forming
- Produces rough, adherent colonies on culture media, often resembling molar tooth shapes
These characteristics help it to colonize the oral cavity efficiently, particularly on tooth surfaces.
Habitat and Ecological Niche
- viscosus is commonly isolated from:
- Dental plaque
- Saliva
- Gingival crevices
- Tonsillar crypts
- Oral mucosa
Its primary habitat is the human mouth, but it may also transiently colonize other mucosal surfaces.
Role in Biofilm Formation
One of the significant roles of A. viscosus is its contribution to biofilm formation on tooth surfaces. It is among the early colonizers that adhere to the dental pellicle and provide a scaffold for other bacteria. This biofilm:
- Promotes plaque development
- Shields bacteria from host immune responses
- Contributes to resistance against antibiotics
- Facilitates interactions with other periodontal pathogens such as Porphyromonas gingivalis and Fusobacterium nucleatum
Pathogenicity and Clinical Relevance
Though A. viscosus is a commensal organism, it can act as an opportunistic pathogen under favorable conditions, particularly when oral hygiene is poor or host immunity is compromised.
1. Dental Diseases
- Dental Plaque: One of the primary bacteria involved in early plaque formation.
- Gingivitis: Found in increased numbers in inflamed gingival tissues.
- Periodontitis: Associated with chronic infection of the periodontal tissues, contributing to alveolar bone loss.
2. Actinomycosis
While A. israelii is the most common cause of actinomycosis, A. viscosus has been implicated in rare cases. Actinomycosis is characterized by:
- Chronic, suppurative infections
- Formation of abscesses and sinus tracts
- Sulfur granules in pus (a diagnostic hallmark)
3. Systemic Infections
Though rare, A. viscosus has been reported in:
- Endocarditis (infection of the heart valves)
- Pulmonary infections, especially in immunocompromised individuals
- Cerebral abscesses and intra-abdominal infections
Diagnosis
Diagnosis of infections caused by A. viscosus typically involves:
- Clinical examination (especially for periodontal disease)
- Gram stain: Reveals gram-positive, filamentous rods
- Culture: Grown under anaerobic or microaerophilic conditions; colonies appear rough and adherent
- Molecular methods: 16S rRNA sequencing or PCR can provide species-level identification
- Histopathology: In actinomycosis, sulfur granules may be observed
Treatment
- viscosus infections respond well to:
- Antibiotics: Penicillin is the drug of choice. Alternatives include amoxicillin, clindamycin, doxycycline, and erythromycin.
- Long-term therapy: Chronic infections like actinomycosis require extended antibiotic courses (weeks to months).
- Surgical intervention: In cases of abscess formation, debridement may be necessary.
- Oral hygiene measures: Prevention of dental diseases through brushing, flossing, and regular dental visits
Prevention
- Good oral hygiene practices
- Regular dental check-ups
- Timely treatment of dental caries and gum diseases
- Limiting sugar intake to reduce substrate for bacterial metabolism
- Use of antimicrobial mouthwashes
Notable Studies and Findings
- Slots & Genco (1983): Demonstrated high prevalence of A. viscosus in patients with gingivitis and periodontitis.
- Nyvad & Kilian (1990): Identified A. viscosus in initial biofilms on root surfaces.
- Brook (2008): Reported its role in rare cases of anaerobic bacteremia and endocarditis.
Conclusion
Actinomyces viscosus is a significant member of the oral microbiome with dual roles as a commensal and opportunistic pathogen. Its ability to form biofilms and interact with other pathogens makes it a key contributor to dental plaque and periodontal diseases. Though systemic infections are rare, they can be serious, warranting early diagnosis and appropriate antibiotic therapy. Understanding its role in health and disease can help in devising effective preventive and therapeutic strategies in oral care.
References
- Slots, J., & Genco, R. J. (1983). Black-pigmented Bacteroides species, Capnocytophaga species, and Actinomyces viscosus in human periodontal diseases. Journal of Clinical Periodontology, 10(4), 422-432.
- Nyvad, B., & Kilian, M. (1990). Microflora associated with experimental root surface caries in humans. Infection and Immunity, 58(5), 1628-1633.
- Brook, I. (2008). The role of anaerobic bacteria in bacteremia. Anaerobe, 14(4), 143-149.
- Finegold, S. M., et al. (2007). Anaerobic infections in humans. Academic Press.
- Wade, W. G. (2013). The oral microbiome in health and disease. Pharmacological Research, 69(1), 137–143.
- Socransky, S. S., & Haffajee, A. D. (1992). The bacterial etiology of destructive periodontal disease: current concepts. Journal of Periodontology, 63(4), 322–331.
- Coykendall, A. L. (1989). Classification and identification of the Actinomyces. Clinical Microbiology Reviews, 2(3), 285–293.
