Essential Dental Microbiology

Dentistry is all about microbiology. The two main dental diseases, dental caries and periodontal disease, result from the interaction between bacteria and the dental tissues.

Microbiology and Prevention of Cross Infection

There are a variety of different tissues in the mouth (teeth, gingivae, mucosa, and periodontal ligament) which provides a complex environment for microorganisms to exploit and inhabit.

The mouth has its own microbial flora formed of hundreds of different species. 

Gram Staining and the Bacterial Cell Wall

Dental caries, periodontal disease, pulpitis, dental abscess and pericoronitis are all caused by multiple oral bacteria working in concert.

When clinical samples are sent to the laboratory for identification, the request is normally for “culture and sensitivity to antimicrobials”. Traditional methods of culture on agar plates have been superceded by automated methods and molecular technique such as PCR (Polymerase Chain Reaction) that identify sequences of genes. However, microbiologists still classify bacteria into Gram positive and Gram negative bacteria.

These two broad groups of bacteria differ in the structure and properties of their bacterial cell wall. This determines their ability to take up blue Gram stain (Gram positive species) or to exclude it, Gram negative species.

Human cells do not have cell walls. So this component of the bacteria becomes an ideal target for antibiotics drugs that damage or inhibit its formation, without adversely affecting human cells and organs.  Knowing if a bacterium is Gram positive or negative can aid in choosing the correct class of antibiotic for treatment.

Pathogens and Opportunistic Microbes

Only a small proportion of bacteria and other microbes are able to cause human disease.  These disease causing microbes are referred to as pathogens.  They usually display a variety of virulence factors that can damage the host directly, trigger a destructive immune response or aid the microbe to survive in the human body or the environment.

Other organisms that cause infection in the mouth are opportunistic pathogens such as the yeast fungus Candida. Opportunistic organisms are normally symbiotic, living in harmony with the host (colonisation).  However, if the person becomes immuno-compromised due to underlying disease affecting the immune systems such as HIV or leukaemia, or by the use of immunosuppressive drugs, then the opportunistic organisms can become pathogenic.

Environmental Organisms and Infection Control

An understanding of the basics of microbiology is also important in preventing and controlling the transmission of infection in the dental surgery. Inanimate surfaces can be a source of infections in the dental surgery.

Surfaces repeatedly touched by hands are often contaminated with pathogens and can act as a vehicle for cross transmission. The longer a pathogen persists on a surface, the longer it may be a source of transmission, therefore endangering a susceptible patient or healthcare worker.

Spore forming bacteria Clostridium difficile that can cause a life threatening diarrhoeal disease, and Mycobacterium tuberculosis that causes tuberculosis, have evolved a highly resistant outer coats that allows the organism to resist desiccation and makes them resistant to many disinfectants. Not surprisingly, these species survive for months on surfaces.

You might not expect that most non-sporing gram-positive bacteria (such as Staphylococcus aureus, including the multi-drug resistant MRSA, Gram negative species such as Escherichia coli that cause  gastrointestinal infections or Pseudomonas aeruginosa commonly found in dental unit waterlines) can also survive for months on surfaces.

Even Blood-borne viruses, such as HBV (hepatitis B virus) or HIV, can persist for more than one week, especially if they dry down onto the surface within a blood or saliva spot.

In summary, the most common healthcare associated pathogens may well survive on surfaces for months and be a continuous source of transmission of infection if no regular preventive surface disinfection is performed in the patient-care area - the dirty zone. Therefore the importance of designing and purchasing easily cleansable equipment and instruments is paramount.

Measures to prevent cross infection in the dental surgery are now part of healthcare legislation. The Care Quality Commission’s benchmark for infection control essential quality requirements are described in “The Health and Social Care Act 2008: Code of Practice on the prevention and control of infections and related guidance (HCAI Code of Practice)”.

Practical methods for implementing the legislation are found in HTM 01-05 Decontamination in Primary Care Dental Practices.