Role of Environment

All bacteria within the oral cavity share the same opportunities for invading the root canal space, however only a restricted group of species have been identified in infected root canals.

The reason for the disproportionate ratio between potential and actual number of species is that the root canal is a unique environment where biological selection drives the type and course of infection. An anaerobic milieu, interactions between microbial factors and the availability of nutrition are principal factors that define the composition of the microbial flora.

In the initial phase of a root canal infection, the number of species is usually low. If the way of invasion is via caries, the bacteria in front of the carious process are the first to reach the pulp. In cases where there is no apparent communication with the oral cavity and the bacteria penetrate through dentinal tubules, as in trauma cases without pulp exposure, there is no clear pattern of primary bacterial invaders.

The number of bacterial species in an infected root canal may vary from one to more than 12, and the number of bacterial cells varies from <102 to >108 per sample. A correlation seems to exist between the size of the periapical lesion and the number of bacterial species and cells in the root canal. Teeth with long-standing infections and large lesions usually harbor more bacterial species and have a higher density of bacteria in their root canals than teeth with small lesions.

Of the varied number of microorganisms only a limited number have been consistently isolated from endodontic infections. These include species of the genera Streptococcus, Fusobacterium, Prevotella, Porphyromonas, Eubacterium, Peptostreptococcus, Bacteroides, and Lactobacillus.

The endodontic milieu is a selective habitat that supports the development of specific proportions of the anaerobic microflora. Oxygen and oxygen products play an important role as ecological determinants in the development of specific proportions of the root canal microflora. The consumption of oxygen and production of carbon dioxide and hydrogen along with the development of a low reduction-oxidation potential by the pioneer species favour the growth of anaerobic bacteria.

Nutrition as an Ecological Driver

The type and availability of nutrients is important in establishing microbial growth. Nutrients may be derived from the oral cavity, degenerating connective tissue, dentinal tubule contents, or a serum-like fluid from periapical tissue. Exogenous nutrients, such as fermentable carbohydrates, affect the microbial ecology of the coronal part of an exposed root canal by promoting growth of species that primarily obtain energy by carbohydrate fermentation. Endogenous proteins and glycoproteins are the principal nutrients in the main body of the root canal system and this substrate encourages the growth of anaerobic bacteria capable of fermenting amino acids and peptides.