Orally administered pathobionts and commensals have comparable and innocuous systemic effects on germ-free mice

Recent evidence suggests that oral bacteria can affect extra-oral diseases by modulating aspects of the gut environment such as the microbiome, metabolome, and immune profiles. However, differences in the effects of different types of oral bacteria, particularly periodontopathic and health-associated bacteria, remain elusive.

Five-week-old germ-free mice were orally administered with either periodontopathic bacteria as oral pathobionts (Porphyromonas gingivalis, Filifactor alocis, and Fusobacterium nucleatum) or bacteria associated with periodontal health (Actinomyces naeslundii, Streptococcus mitis, and Veillonella rogosae) twice a week for five weeks. The presence of all bacterial species in the feces and the livers of the mice was analyzed via polymerase chain reaction (PCR), using specific primers for 16S rRNA genes. Alveolar bone resorption was evaluated histologically. The expression profiles of various genes in the liver and small intestine were analyzed using real-time PCR. Sera were analyzed to determine the levels of antibodies and endotoxin. The proportions of T helper 17 (Th17) and regulatory T (Treg) cells in mesenteric lymph nodes and Peyer's patches were analyzed using flow cytometry.

Neither of the types of bacteria administered in this experiment induced alveolar bone resorption. All bacteria elicited some degree of systemic antibody response in the mice, although the response to S. mitis was not obvious. The response to P. gingivalis and V. rogosae was strongest. Generally, the health-associated bacteria but not the periodontitis-associated bacteria were detected in fecal samples. Interestingly, only Fusobacterium nucleatum DNA was detected in the liver, despite that live Fusobacterium nucleatum were not detected in the liver. The levels of interleukin-17 in the intestine and genes related to lipid accumulation in the liver were significantly higher in the mice that received periodontitis-associated bacteria. In addition, expression of the gene associated with endoplasmic reticulum stress was higher and that of the gene controlling circadian rhythm was lower in the periodontitis group. There was no difference in serum endotoxin, T-cell phenotypes in the lymphatic tissues, or genes related to the gut barrier.

Oral administration of periodontitis-associated bacteria can induce pathological changes in the liver and intestine that are implicated in the process of periodontitis. These findings further support the importance of the oral-gut connection.