Microbiology, Immunology, and Infection Control
What this domain tests
Questions cover the plaque biofilm and its key organisms, host immune responses relevant to caries and periodontitis, and the practical standards of sterilisation, disinfection and post-exposure management.
Microbiology and immunology
- Caries: Streptococcus mutans initiates, using glucosyltransferase to make water-insoluble glucan for adherence; lactobacilli dominate advancing lesions; Actinomyces species associate with root caries.
- Periodontitis: the red complex is Porphyromonas gingivalis, Tannerella forsythia and Treponema denticola. Aggregatibacter actinomycetemcomitans produces leukotoxin and associates with stage III/IV grade C disease in young patients. Fusobacterium nucleatum acts as a bridge organism.
- Host response: neutrophils dominate the gingival crevice; IgG passes into crevicular fluid while secretory IgA dominates saliva; the RANKL/OPG ratio governs osteoclast activation and bone loss.
- Viruses of dental relevance include herpes simplex, varicella-zoster, hepatitis B and C, HIV, HPV and prions; standard precautions apply to all patients regardless of known status.
Infection control standards
- Hand hygiene at the WHO five moments; gloves, mask, eye protection and gown as indicated.
- Sterilisation: autoclave at 134 °C for 3 minutes or 121 °C for 15 minutes; class B cycles for wrapped, hollow and porous loads; Geobacillus stearothermophilus spore tests for steam, weekly or per policy; chemical indicators only confirm exposure, not sterility.
- Instrument processing: clean, inspect, pouch, date, sterilise, store; traceability records.
- High-level disinfection with glutaraldehyde or ortho-phthalaldehyde for heat-sensitive items; surface barriers preferred to sprays.
- Dental unit waterlines should deliver no more than 500 CFU/mL; flush lines and use independent reservoirs.
Worked example
A hygienist sustains a needlestick from a patient of unknown status. Reasoning: wash the wound with soap and water, do not scrub or suck; report immediately; source and exposed staff tested for HIV, HBV and HCV. HIV post-exposure prophylaxis should start within 2 hours, ideally, using a three-drug regimen for 28 days. For hepatitis B, check vaccination and anti-HBs titre; give vaccine plus immunoglobulin if non-immune. Hepatitis C has no prophylaxis, so monitor and treat if seroconversion occurs.
Traps
Believing a negative history removes the need for precautions; confusing disinfection with sterilisation; using chemical indicators as proof of sterility; forgetting that chlorhexidine and sodium hypochlorite must never be mixed.