Evidence indicates that major depression is accompanied by increased 
translocation of gut commensal Gram-negative bacteria (leaky gut) and consequent 
activation of oxidative and nitrosative (O&NS) pathways. This present study 
examined the associations among chronic apical periodontitis (CAP), root canal 
endotoxin levels (lipopolysaccharides, LPS), O&NS pathways, depressive symptoms, 
and quality of life. Measurements included advanced oxidation protein products 
(AOPP), nitric oxide metabolites (NOx), lipid peroxides (LOOH), -sulfhydryl (SH) 
groups, total radical trapping antioxidant parameter (TRAP), and paraoxonase 
(PON)1 activity in participants with CAP, with and without depression, as well 
as healthy controls (no depression, no CAP). Root canal LPS levels were 
positively associated with CAP, clinical depression, severity of depression (as 
measured with the Hamilton Depression Rating Scale (HDRS) and the Beck 
Depression Inventory) and O&NS biomarkers, especially NOx and TRAP. CAP-related 
depression was accompanied by increased levels of NOx, LOOH, AOPP, and TRAP. In 
CAP participants, there was a strong correlation (r = 0.734, p < 0.001) between 
root canal LPS and the HDRS score. There were significant and positive 
associations between CAP or root canal endotoxin with the vegetative and 
physio-somatic symptoms of the HDRS as well as a significant inverse association 
between root canal endotoxin and quality of life with strong effects on 
psychological, environmental, and social domains. It is concluded that increased 
root canal LPS accompanying CAP may cause depression and a lowered quality of 
life, which may be partly explained by activated O&NS pathways, especially NOx 
thereby enhancing hypernitrosylation and thus neuroprogressive processes. Dental 
health and "leaky teeth" may be intimately linked to the etiology and course of 
depression, while significantly impacting quality of life.