Properties of species associated with persistent endodontic disease

With the exception of Actinomyces, which is primarily

involved in extraradicular infection, other species

commonly associated with persistent intraradicular

infection such as candida and enterococci can be  viewed

as opportunistic pathogens. A shared behaviour is that

they leave their normal habitat in the oral cavity and

establish in the root canal where they take advantage of

the ecological changes and that their microbial

competitors have been eliminated by treatment.

For microbes to maintain apical periodontitis and

continue to cause disease, they must do more than just

survive in the root-filled canal; they must also possess

the pathogenic properties necessary to perpetuate

inflammation external to the root canal system. In

general, micro-organisms involved in persistent

infections implement one of three strategies to evade

the immune response — sequestration, cellular or

humoral evasion.

Sequestration involves a physical

barrier between the microbe and the host. Cellular

evasion means that micro-organisms avoid leukocyte

dependent antibacterial mechanisms. Humoral evasion

means that those extracellular bacteria avoid the host’s

antibodies and complement.

At least two of the three strategies are deployed by

micro-organisms involved in persistent endodontic

disease.

A. israelii is an example of an endodontic

pathogen that displays cellular evasion by avoiding

phagocytosis by PMN leukocytes  in vivo

primarily

through a mechanism of collective cohesion.

E. faecalis and Candida species are representative of

microbes that can remain sequestered within the root

canal system.

Micro-organisms involved in persistent endodontic

disease require a range of properties that allow them to

enter and establish in the root canal, survive the

antimicrobial treatment and induce or maintain apical

periodontitis (Fig 4). 

That low numbers of cells survive

endodontic treatment implies an ability of some species

to withstand instrumentation and antimicrobial

irrigation, however numerous reports confirm the

bactericidal efficacy of sodium hypochlorite against

species involved in persistent infection such as  A.

israelii ,

E. faecalis

and Candida.

Thus, these

species may have the capacity to shelter from the main

root canal in web-like areas, canal ramifications or

dentinal tubules where some level of protection or

buffering of the antimicrobial agent is possible.

Although most root canal bacteria are sensitive to the

high pH of calcium hydroxide,

several species

involved in persistent infection are known to have a

capacity to resist a high pH.

How bacteria endure root filling is unknown, but

studies that have sampled the root canal prior to root

filling and then followed the treatment outcome of

infected teeth have shown that some lesions heal,

implying that the bacteria did not survive or were not

able to inflame the periapical tissue. Whether or not

bacteria survive root filling may depend on whether

they are entombed, or blocked from acquiring

nutrition. It is possible, even likely, that bacteria may

undergo a period of starvation. The ability of 

E. faecalis to endure periods of starvation,

is a trait that may be crucial for survival.
Apical periodontitis is a dynamic process involving
an interaction between host and living bacteria, and the
microbes need to find substrates for growth. In a well-instrumented root canal where necrotic pulp tissue has
been removed and there is no communication with
exogenous nutrients from the oral cavity, nutrition is
likely to come from a periapical fluid transudate, which
is probably serum-like in nature. The capacity of some
species to degrade serum and tissue molecules
corresponds with an ability to avoid the host defence
and induce an inflammatory response.
An ability to utilize collagen within dentine may also be useful and
there are indications that  E. faecalis may have this

property.