Relative reduction of biological and phylogenetic diversity of the oral microbiota of diabetes and pre-diabetes patients.

•The reciprocal relationship between diabetes and oral health has been supported by multiple lines of evidence.•We compared the oral microbiomes, to the species level, in normoglycemic, glucose intolerant, and diabetes patients.•A clear reduction of the biological and phylogenetic diversity was apparent in diabetes and pre-diabetes oral microbiomes.•This was associated with an increase in the pathogenic content of the hyperglycemic microbiome.•The results of this study may aid to better understanding of the directionality of the mysterious reciprocal relationship.