Actinomyces naeslundii Morphology, Pathogenic Role, and Clinical Implications in Oral and Systemic Health

Introduction

Actinomyces naeslundii is a Gram-positive, non-spore-forming, facultatively anaerobic bacterium that is a key member of the human oral microbiome. It was first described by Hamada in 1970 and is part of the genus Actinomyces, which includes other medically relevant species such as A. israelii, A. viscosus, and A. odontolyticus. Despite being a commensal organism, A. naeslundii has been associated with a number of oral and systemic infections, particularly when normal microbial balance is disturbed. This article discusses the morphology, ecology, pathogenic potential, clinical significance, and current research trends concerning Actinomyces naeslundii.

Morphological and Physiological Characteristics

Actinomyces naeslundii is a filamentous, branching rod-shaped bacterium. It is:

• Gram-positive: Due to a thick peptidoglycan layer
• Non-motile and non-spore-forming
• Facultative anaerobe: It grows in both aerobic and anaerobic conditions, though it prefers low oxygen environments.
• Catalase-negative

On blood agar, A. naeslundii colonies appear as “molar tooth”-shaped—typical of many Actinomyces species. It adheres strongly to surfaces such as enamel, epithelial cells, and other bacteria through fimbriae and surface adhesins, contributing to biofilm formation in the oral cavity.

Ecological Niche and Commensal Role

Actinomyces naeslundii is part of the early colonizers of dental biofilms. It inhabits:

• Oral mucosal surfaces
• Tooth surfaces (supragingival and subgingival plaque)
• Saliva and tongue dorsum

It plays a role in:

• Maintaining oral microbiota balance
• Coaggregation with other bacteria, e.g., Streptococcus gordonii, Fusobacterium nucleatum
• Contributing to plaque maturation and biofilm architecture
  

While often beneficial, disruption in oral hygiene or immunity can shift A. naeslundii from a commensal to an opportunistic pathogen.

Pathogenic Potential

Though less virulent than A. israelii, A. naeslundii can become pathogenic under certain conditions, especially in immunocompromised individuals or those with poor oral hygiene.

1. Dental Caries and Root Surface Caries

1. naeslundii is highly associated with:

• Early enamel colonization
• Acid production through carbohydrate fermentation, contributing to root caries
• Collaboration with Lactobacillus and Streptococcus mutans in demineralization

2. Periodontal Disease

• Found in high numbers in subgingival plaque of patients with chronic periodontitis
• Can initiate inflammatory responses and tissue degradation

3. Actinomycosis

• Though rare, A. naeslundii has been isolated from cervicofacial actinomycosis cases
• Can form sulfur granules and abscesses

4. Endodontic Infections

• Infected root canals may harbor A. naeslundii, contributing to periapical lesions and treatment resistance

5. Systemic Infections

In rare cases, it can spread hematogenously leading to:

• Lung abscesses
• Osteomyelitis
• Endocarditis and brain abscesses in vulnerable patients

Virulence Factors

• Adhesins: Fimbriae mediate attachment to tooth and epithelial surfaces
• Biofilm formation: Helps evade immune responses and antimicrobials
• Acidogenicity and aciduricity: Tolerates and produces acidic environments, relevant in caries
• Immune evasion: Resists phagocytosis and promotes chronic granulomatous inflammation

Diagnosis

Identification of A. naeslundii involves:

• Microscopy: Gram-positive branching rods
• Culture: Grows slowly on anaerobic media
• Biochemical tests: Carbohydrate fermentation profiles
• Molecular methods: 16S rRNA gene sequencing and MALDI-TOF MS have improved species-level identification

In tissue infections, histology may reveal sulfur granules surrounded by neutrophils and macrophages.

Antibiotic Sensitivity and Treatment

1. naeslundii is generally sensitive to:

• Penicillin (first-line)
• Amoxicillin, clindamycin, cephalosporins
• Tetracyclines

Resistance to metronidazole is common due to its poor activity against facultative anaerobes. For actinomycosis, long-term antibiotic therapy (4–6 weeks) may be required, often combined with surgical debridement.

Clinical Implications and Prevention

Dental context:

• Regular dental hygiene, fluoride use, and plaque control reduce colonization and risk of disease
• Chlorhexidine rinses may help suppress Actinomyces in plaque

Systemic risk:

• Immunocompromised individuals, patients with prosthetic devices, or those undergoing dental procedures should be monitored for invasive infections

Current Research Trends

Recent studies have focused on:

• Biofilm dynamics: Understanding multispecies interaction with Streptococcus and Fusobacterium
• Molecular identification: Use of next-generation sequencing to detect A. naeslundii in microbiome studies
• Role in systemic diseases: Its DNA has been detected in atherosclerotic plaques and aspiration pneumonia cases

Conclusion

Actinomyces naeslundii plays a dual role in human health: it contributes to oral microbial homeostasis but can also act as an opportunistic pathogen. Understanding its behavior in both health and disease states is essential for developing effective preventive and therapeutic strategies. Improved molecular techniques have shed light on its complex interactions and role in polymicrobial communities, reinforcing the need for targeted interventions in dental and systemic care.

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