15.1 Oral Microbiology & Infection Control
Key Takeaways
- Dental plaque is a structurally organised biofilm; Streptococcus mutans and other mutans streptococci initiate caries, while Porphyromonas gingivalis, Tannerella forsythia and Aggregatibacter actinomycetemcomitans are key periodontopathogens.
- Dental unit water lines (DUWLs) harbour biofilms of opportunistic environmental bacteria (e.g. Pseudomonas, Legionella); output must meet potable-water standards (≤200 CFU/mL) using independent reservoirs, filters, and flushing.
- Standard (universal) precautions apply to all patients: hand hygiene, gloves, masks, eye protection, and safe handling of sharps — because patients may be unknowingly infectious.
- HTM 01-05 (England) and equivalent UK guidance define decontamination: cleaning, disinfection, and sterilisation of reusable instruments, with validation and traceability.
- Sterilisation is by autoclave (moist heat, 134–137 °C for 3 min, or 121–124 °C for 15 min); prion-instrument risk (endodontic files) requires single-use where possible.
Dental Plaque as a Biofilm
Dental plaque is not random debris — it is a structurally organised biofilm of cooperating microbial communities embedded in an extracellular polymeric matrix. The biofilm protects its members from antimicrobials and host defences; this is why mechanical disruption (brushing, interdental cleaning, professional debridement) is more effective than antimicrobials alone.
Plaque Formation Stages
- Acquired pellicle — glycoproteins from saliva coat the clean tooth within minutes.
- Initial colonisation — Streptococcus oralis, S. sanguinis, S. mitis adhere (primary colonisers).
- Co-aggregation and bridging — fusobacteria bridge early Gram-positive cocci to later colonisers.
- Maturation — anaerobes, motile rods, and spirochaetes dominate the mature subgingival biofilm.
Microorganisms of Caries
| Organism | Role |
|---|---|
| Streptococcus mutans (and other mutans streptococci) | Primary caries initiator; acidogenic, aciduric; metabolises sucrose to extracellular polysaccharide (glucan) |
| Lactobacillus species | Aciduric; drive lesion progression in dentine |
| Actinomyces species | Root caries |
S. mutans characteristics: ferments sucrose to lactic acid, survives at low pH (aciduric), synthesises glucans via glucosyltransferases that aid adhesion. The critical pH (~5.5) is the pH at which enamel demineralises; it relates to the saturation of the saliva with respect to hydroxyapatite.
Microorganisms of Periodontal Disease
Current consensus groups periodontopathogens as the red complex (Porphyromonas gingivalis, Tannerella forsythia, Treponema denticola) and the orange complex (Fusobacterium, Prevotella, Campylobacter) that precedes it.
| Organism | Association |
|---|---|
| Aggregatibacter actinomycetemcomitans | Localised aggressive periodontitis |
| Porphyromonas gingivalis | Chronic and aggressive periodontitis; key virulence (gingipains) |
| Tannerella forsythia | Red complex; chronic periodontitis |
| Treponema denticola | Spirochaete; deep subgingival biofilm |
| Prevotella intermedia | Pregnancy-associated gingivitis; anaerobe |
Virulence factors: proteases (gingipains of P. gingivalis degrade host tissues and immune molecules), lipopolysaccharide (LPS), fimbriae (adhesion), and leukotoxin (A. actinomycetemcomitant kills neutrophils).
Other Key Organisms
- Candida albicans — dimorphic yeast; oral candidiasis, especially in denture wearers and the immunocompromised; pseudohyphae on smear.
- Herpes simplex virus (HSV-1) — primary herpetic gingivostomatitis; recurrent as cold sores.
- Group B streptococci / S. pyogenes — acute necrotising ulcerative gingivitis involves spirochaetes and fusobacteria (Borrelia/Treponema + Fusobacterium), not streptococci.
Infection Control — Standard (Universal) Precautions
Because patients may be unknowingly infectious (HIV, hepatitis B/C), standard precautions apply to every patient:
- Hand hygiene before and after each patient (WHO 5 moments): before touching a patient, before clean/aseptic procedure, after body fluid exposure risk, after touching a patient, after touching patient surroundings.
- Personal protective equipment (PPE) — gloves, surgical mask, eye protection, and a tunic/apron.
- Sharps safety — single-handed recapping avoided; sharps to a container at point of use; never resheath a needle.
- Needlestick management — encourage bleeding, wash with soap and water, seek occupational health advice and post-exposure prophylaxis (PEP) assessment urgently (within hours) for HIV; hepatitis B booster if indicated.
Immunisation
- Hepatitis B vaccination is a requirement for clinical dental staff; check antibody titre; non-responders are identified and managed.
- TB (BCG) status and other occupational immunisations as per local policy.
Dental Unit Water Lines (DUWLs)
DUWLs develop biofilms of opportunistic environmental bacteria (Pseudomonas aeruginosa, Legionella species, Mycobacterium species). Exposure to contaminated aerosols has caused infection in patients and staff.
- Standard: output water should meet potable-water limits — ≤200 colony-forming units (CFU)/mL (HTM 01-05).
- Control measures: independent water reservoirs, point-of-use filters, chemical treatment, and flushing lines for 2 minutes at the start of the day and 20–30 seconds between patients.
- Use of sterile water/saline for surgical procedures.
Decontamination — HTM 01-05 and the Decontamination Cycle
HTM 01-05 (Health Technical Memorandum, England; equivalent guidance elsewhere in the UK) defines essential quality requirements and best practice for dental instrument decontamination.
The Decontamination Cycle
- Cleaning — removes soil; essential because disinfectant/sterilant cannot penetrate organic matter. Manual or washer-disinfector (preferred, validated).
- Disinfection — reduces microbial load but not spores; for surfaces, impressions, and some equipment.
- Sterilisation — destroys all microorganisms including spores.
- Storage — packaged, dated, kept dry; shelf life by packaging.
Sterilisation Parameters
| Method | Parameters | Use |
|---|---|---|
| Moist heat autoclave (vacuum) | 134–137 °C for ≥3 min | Wrapped instruments; preferred (porous load) |
| Moist heat autoclave (gravity) | 121–124 °C for ≥15 min | Unwrapped, non-porous |
| Dry heat | 160–180 °C | Rare; limited use |
| Chemical vapour | Limited use |
Prion (vCJD) Risk
Prions are resistant to standard autoclaving. Endodontic files are considered higher-risk because they contact neural/lymphoid tissue (pulp) and cannot be reliably cleaned; guidance is to make them single-use wherever possible. Other risk instruments (e.g. surgical instruments contacting lymphoid tissue) require stricter handling; a prion risk assessment categorises instruments.
Validation and Traceability
Best practice requires validation of autoclave cycles, regular testing (helix/Bowie-Dick test), and traceability — each instrument set linked to the patient and steriliser cycle, so an incident can be traced to its source.
Which organism is most strongly associated with molar-incisor pattern periodontitis in adolescents (formerly localised aggressive periodontitis), producing a leukotoxin that impairs neutrophils?
At what pH does enamel begin to demineralise — the so-called critical pH — because saliva becomes undersaturated with respect to hydroxyapatite?
After a needlestick injury from a known hepatitis B-infected source, what is the recommended first action followed by urgent occupational health review?
Which statement about endodontic files and decontamination under current UK guidance is correct?
Under HTM 01-05, the output of a dental unit water line used for non-surgical treatment should contain no more than which concentration of microorganisms?