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, inter-actions 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.
bacterial species in an infected root canal may vary
from one to more than 12, and the number of bacterial
cells varies from 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 harbour more bacterial species and have
a higher density of bacteria in their root canals than
teeth with small lesions.
Studies on the dynamics of root canal infections have
shown that the relative proportions of anaerobic micro-organisms and bacterial cells increase with time and
that the facultatively anaerobic bacteria are out-numbered when the canals have been infected for three
months or more.
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.