Microbial Ecology and Biofilms

The microflora ecosystem involved in root canal is appropriately described as ‘polymicrobial’ in nature.

In carious exposed root canals most commonly found facultative anaerobic species are alpha-hemolytic streptococci, staphylococci, lactobacilli. Large portion of microorganisms isolated from intact non-vital teeth are anaerobes e.g. Bacteriodes, peptococcus, peptostreptococcus, fusiform.

Strict anaerobes — These bacteria grow only in the absence of oxygen but vary in their sensitivity to oxygen. They function at low oxidation-reduction potentials and generally lack the enzymes superoxide dismutase and catalase. Prevotella nigrecens is the dark pigmented bacteria most often cultivated from endodontic infections. Microaerophilic bacteria can grow in an environment with oxygen but redominantly derive their energy from anaerobic energy pathways.

Facultative anaerobes grow in the presence or absence of oxygen and usually have the enzymes superoxide dismutase and catalase.

Obligate aerobes require oxygen for growth and possess both superoxide dismutase and catalase.

The interdependence of different bacterial species with their environment is the key to the success of root canal treatment. The treatment procedures (mechanical and chemical) essentially interfere with the environment, killing some bacteria and indirectly killing other species by altering the nutritional and toxic balance.

The surviving bacteria are usually those hardy enough to resist the treatment and capable of living independently of other species in unique nutrition-depleted conditions. This means a poor first attempt at root canal treatment may result in a more resistant and hardy infection to eradicate at the next step. It is therefore best to launch the most comprehensive effort at eradicating the infection at the first attempt.

Bacteria in a root canal infection do not occur in vivo as separate colonies, but grow within an extracellular matrix in interconnected communities as a ‘bacterial biofilm’. The ultrastructural appearance of these biofilms in the infected root canal is described as coaggregating communities with a palisade structure.

The clinical significance of a biofilm growth pattern is that bacteria are relatively protected within the coaggregated community compared with planktonic forms and are known to be more resistant to antimicrobial treatment measures. This is because of the several mechanisms, including the following:

• the exopolysaccharide in which the bacteria are embedded may restrict diffusion of the antibacterial agents to the cells;
• the different layers of cells similarly act as barriers to diffusion;
• some bacterial cells may be slower-growing or dormant and therefore may be more resistant to killing;
• cells may exhibit specific resistant mechanisms;
• biofilm phenotypes may be inherently resistant.

Currently, limited information is available on the development, physiology and antimicrobial management of biofilms in the root canal.