24.2 Microbial Diseases of the Mouth and Oral Cavity

Microbial Diseases of the Mouth and Oral Cavity

The mouth is one of the most densely colonized sites in the human body, harboring hundreds of microbial species. When the balance of this community shifts, bacteria and fungi can cause diseases ranging from tooth decay to severe bone loss around the teeth. This section covers the major oral pathogens, how they cause disease, and how oral infections are prevented and treated.

Causes and Progression of Dental Diseases

Dental caries (tooth decay) is caused by acid-producing bacteria, primarily Streptococcus mutans and Lactobacillus species. These organisms metabolize dietary sugars (sucrose, fructose) and release acid as a metabolic byproduct. That acid demineralizes tooth enamel, eventually producing cavities.

Caries progress through a predictable sequence:

1. Initial demineralization of the enamel surface (often visible as a white spot)
2. Enamel decay and formation of small cavities
3. Dentin decay as bacteria penetrate deeper into the softer tissue beneath the enamel
4. Pulp infection if decay reaches the inner core of the tooth, which contains nerves and blood vessels

Periodontal disease involves inflammation and infection of the gums (gingivae) and the structures that anchor teeth in place (periodontal ligament and alveolar bone). It's driven by the buildup of dental plaque, a bacterial biofilm that accumulates along the gumline. The key pathogens are Porphyromonas gingivalis, Tannerella forsythia, and Treponema denticola.

Periodontal disease also follows a staged progression:

1. Gingivitis — inflammation of the gums (redness, swelling, bleeding) without any bone loss. This stage is reversible with proper hygiene.
2. Periodontitis — gum recession begins, the periodontal ligament breaks down, and alveolar bone is lost. Pockets form between the tooth and gum.
3. Advanced periodontitis — severe bone loss leads to tooth mobility and, eventually, tooth loss.

Key Oral Pathogens and Infections

Streptococcus mutans is the primary cause of dental caries. It produces extracellular polysaccharides called glucans that help bacteria stick to tooth surfaces and build up dental plaque. Once attached, it ferments sugars to produce lactic acid, which directly demineralizes enamel. Its ability to both adhere and acidify makes it especially effective at initiating decay.

Lactobacillus species are secondary colonizers in dental caries. They don't start the process, but they thrive in the acidic environments that S. mutans creates. Once established, they produce additional acids that accelerate demineralization deeper into the tooth structure.

Porphyromonas gingivalis is considered a keystone pathogen in periodontal disease. "Keystone" means it has an outsized effect on the microbial community relative to its abundance. It produces virulence factors including gingipains (proteases that destroy tissue) and lipopolysaccharide (LPS, which triggers a strong inflammatory response). It also modulates the host immune response in ways that allow persistent infection and chronic inflammation rather than clearance of the bacteria.

Tannerella forsythia and Treponema denticola work synergistically with P. gingivalis. Together, these three species form the "red complex", a group strongly associated with severe periodontal disease. They produce proteases that break down connective tissue and contribute to alveolar bone loss.

Candida albicans is a fungal pathogen responsible for oral thrush (oropharyngeal candidiasis). It's a normal part of the oral flora in many people, but overgrowth occurs when conditions shift in its favor. Common risk factors include immunosuppression (HIV/AIDS), prolonged antibiotic use (which reduces competing bacteria), and denture wear. Oral thrush presents as white, curd-like patches on the tongue, palate, and inner cheeks (buccal mucosa) that can be scraped off to reveal reddened tissue underneath.

Oral Microbiome and Host Interactions

The oral microbiome is a diverse community of bacteria, fungi, viruses, and even protozoa living in the mouth. In a healthy state, these organisms coexist in a balanced ecosystem that actually helps prevent disease by occupying niches and outcompeting potential pathogens. Problems arise when this balance is disrupted, a concept called dysbiosis.

Biofilm formation is central to oral disease. Microorganisms adhere to tooth surfaces, multiply, and build structured communities encased in a matrix of polysaccharides and proteins. Dental plaque is the classic example. Bacteria within biofilms are far more resistant to antimicrobials and immune defenses than free-floating (planktonic) bacteria, which is why plaque removal through mechanical means (brushing, flossing) is so important.

Dental anatomy influences where microbes colonize. Enamel, dentin, and cementum each have different surface properties. Pits and fissures on the chewing surfaces of molars are particularly vulnerable to bacterial accumulation because they're difficult to clean.

Saliva plays a major protective role. It contains antimicrobial compounds (lysozyme, lactoferrin, secretory IgA) that help regulate microbial populations. It also acts as a buffer, neutralizing acids produced by bacteria and helping to maintain a pH that protects enamel from demineralization. Reduced saliva flow (xerostomia) significantly increases the risk of caries.

The host immune response in the oral cavity involves both innate defenses (neutrophils, antimicrobial peptides in gingival crevicular fluid) and adaptive immunity (secretory IgA, T cells). In periodontal disease, the immune response itself contributes to tissue damage. Chronic inflammation driven by bacterial persistence leads to the release of cytokines and enzymes that destroy the periodontal ligament and bone. This is why periodontal disease is sometimes described as an immunopathological condition: the host's own immune response causes much of the damage.

Prevention and Treatment of Oral Diseases

Prevention Strategies

Preventing dental caries and periodontal disease centers on controlling plaque and limiting the substrates bacteria use to produce acid:

• Brushing twice daily with fluoride toothpaste
• Flossing daily to remove plaque from between teeth where brushes can't reach
• Antiseptic mouthwash (e.g., chlorhexidine) to reduce overall bacterial load
• Regular dental check-ups with professional cleaning (scaling and polishing) to remove hardened plaque (calculus) that can't be removed at home
• Limiting sugary and acidic foods and drinks (soda, candy, fruit juices), since these provide the substrates for bacterial acid production
• Fluoride treatments to strengthen enamel and promote remineralization of early lesions
• Dental sealants applied to the pits and fissures of molars to physically block bacteria from colonizing these vulnerable surfaces

Treatment Strategies

Dental caries treatment:

• Removal of decayed material and restoration of the cavity with filling materials (composite resin, amalgam)
• Root canal therapy for advanced cases where decay has reached the pulp; the infected pulp tissue is removed, the canal is disinfected, and the tooth is sealed to prevent reinfection

Periodontal disease treatment:

• Scaling and root planing — a deep-cleaning procedure that removes plaque, calculus (tarite), and bacterial toxins from tooth surfaces and within periodontal pockets
• Antibiotics such as metronidazole or doxycycline to control bacterial infection and reduce inflammation, often used as an adjunct to mechanical debridement
• Surgical interventions for advanced cases, including flap surgery (to reduce pocket depth and allow better access for cleaning) and bone grafts (to regenerate lost alveolar bone)

Oral thrush treatment:

• Antifungal medications such as nystatin oral suspension or fluconazole tablets
• Addressing underlying causes: improving oral hygiene, disinfecting dentures regularly, and treating the conditions (e.g., immunosuppression) that allowed fungal overgrowth

When selecting antimicrobial treatments for any oral infection, antimicrobial resistance is an important consideration. Overuse of antibiotics can select for resistant strains, so treatment should be targeted based on the specific pathogen and clinical situation.