Microbiology, Immunology, and Infection Control
Scope
Covers the oral microbiome and the specific pathogens of caries and periodontitis, the principles of sterilisation and disinfection, and standard precautions against blood-borne viruses.
Microbiology
- Caries: mutans streptococci initiate, lactobacilli propagate, Actinomyces species dominate root caries; the Stephan curve shows the pH fall after sugar and its salivary recovery.
- Periodontitis: the red complex - Porphyromonas gingivalis, Treponema denticola, Tannerella forsythia; Aggregatibacter actinomycetemcomitans in aggressive forms.
Spaulding classification
- Critical items enter sterile tissue or bone (extraction forceps, surgical burs): sterilise.
- Semicritical items contact intact mucosa (mirrors, impression trays, amalgam condensers): sterilise, or high-level disinfection if heat-labile.
- Non-critical items contact intact skin only: low-level disinfection.
Sterilisation specifics
- Steam autoclave: 121 °C at 15 psi for 15–20 minutes, or 134 °C for 3–4 minutes.
- Biological indicator: Geobacillus stearothermophilus spores. The Bowie-Dick test checks vacuum performance, not sterility.
- 2% glutaraldehyde gives high-level disinfection in about 20 minutes; ortho-phthalaldehyde is an alternative.
Worked example
An extraction forceps and a mouth mirror are used on one patient. Reasoning: forceps contact bone and blood, so they are critical and must be autoclaved; the mirror contacts only intact mucosa, so it is semicritical and sterilisation is preferred, with high-level disinfection acceptable only if it is heat-labile.
Traps
- Describing glutaraldehyde as a cold sterilant for critical items - in routine use it is a high-level disinfectant.
- Wiping a handpiece with alcohol between patients instead of sterilising it.
- Forgetting that hepatitis B survives longer on surfaces and is more transmissible per needlestick than HIV.