Dissemination of Oral Bacteria into the Brain in a Ligature Induced-periodontitis Mouse Model

Objectives: Oral bacteria may be involved in the pathogenesis of Alzheimer’s disease (AD). Our preliminary data suggested that the alpha diversity of the microbiome could be increased in the brains of mice with AD. Based on this observation, we hypothesized that the inflammatory response in periodontitis allows bacteria to cross the blood-brain barrier (BBB) and infect the brain. To test this hypothesis, an experimental mouse model of periodontal disease was used to determine whether viable oral bacteria could migrate into mouse brain tissue.
Methods: Periodontal disease was induced in 4 FVB mice, aged 10 weeks, by using a 7–0 silk ligature tied around the maxillary left second molar. Ligatures were applied to both sides of the maxillary dentition. The ligatures remained in place for 4 weeks. The ligatured mice (L1-L4) were sacrificed, and the ligatures and brain tissues were harvested for bacterial culture under aerobic and anaerobic conditions. Isolates from ligature and brain cultures were identified using 16S rRNA gene sequencing.
Results: Isolates were obtained from brains of 3 of the 4 ligatured mice. We found 5 aerobic and 1 anaerobic isolates from L4 brain culture, 5 aerobic and 9 anaerobic isolates from L3 brain culture, and 1 aerobic isolate from L1 brain culture. From these cultures, six bacterial species, all previously described in the oral cavity of mice, were identified as follows: Streptococcus sp. LysM4, Enterococcus faecalis, Micrococcus aloeverae, Streptococcus mitis, Staphylococcus warneri, Gemella haemolysans. In one of the ligatured mice, 2 of the species isolated from the brain, Streptococcus sp. LysM4 and Enterococcus faecalis, were also found in the ligature cultures.
Conclusions: These findings suggested that periodontal disease would lead to the dissemination of oral bacteria into the brain.