Endodontic Microbiology, Inflammation, and Host Response
Biofilm, not planktonic contamination
Root canal infection is a polymicrobial biofilm, dominated in primary infection by obligate anaerobes - Prevotella, Porphyromonas, Fusobacterium, Treponema, Dialister - with facultative organisms such as Enterococcus faecalis, streptococci and lactobacilli persisting in secondary or refractory cases. Candida albicans is the main fungus and is found more often after failed treatment. Biofilms adhere to dentinal walls, invade tubules to several hundred micrometres, and tolerate antiseptics far better than planktonic cells.
Host response
The periapical lesion is an immune response to escaping antigens, especially lipopolysaccharide and lipoteichoic acid. Neutrophils and macrophages release IL-1, TNF-alpha, prostaglandin E2 and RANKL; RANKL with M-CSF drives osteoclast differentiation, while osteoprotegerin neutralises RANKL. This explains why symptoms and lesion size follow the balance of microbial challenge and host defence, and why a large lesion may be asymptomatic.
What kills microbes in the canal
Mechanical instrumentation removes the bulk of the biofilm, but no instrument reaches untouched walls, so chemical irrigation is essential. Sodium hypochlorite dissolves organic tissue, saponifies fats, neutralises toxins and is bactericidal - concentration and volume both matter; higher concentrations dissolve tissue faster but are more cytotoxic. EDTA (17%) removes the smear layer, opening tubules for the next hypochlorite refresh; chlorhexidine is a useful final rinse but forms a precipitate with hypochlorite. Calcium hydroxide medicament has limited penetration and is not a substitute for irrigation.
Traps
- The myth that a resistant organism always explains failure: most failures reflect inadequate disinfection, missed anatomy or leakage.
- Routine culture and sensitivity is not standard of care.
- Extraradicular infection (Actinomyces) and true cysts do not respond to canal treatment.
Rule of thumb: the canal is a space to be cleaned, not merely filled - irrigant delivered and agitated against walls outweighs instrument choice.