Recently, liver disease caused by excess nutrition intake in individuals who do not drink alcohol,so-called non-alcoholic fatty liver disease, has received attention. In affected patients, advanced pathological changes related to inflammation and fibrosis in the liver are termed non-alcoholic steatohepatitis (NASH). Examinations of saliva specimens collected from NASH patients have frequently detected Porphyromonas gingivalis, a major periodontitis-related bacterium. Furthermore, analysis of NASH model mice fed a high-fat diet revealed that infection with P. gingivalis OMZ314, highly virulent for periodontitis, resulted in development of NASH conditions after only 8 weeks, while those model mice otherwise typically develop such conditions after at least 48 weeks. As for cariogenic bacteria,infection with the blood isolate Streptococcus mutans TW871 led to development of NASH within 8 weeks in the same model. Several in vitro analyses also revealed excess production of metallothionein, a cytokine related to oxidative stress, in TW871-infected mice, while inflammatory cytokine and chemokine levels were also elevated, which may be related to aggravation of NASH.