Porphyromonas gingivalis and Treponema denticola are synergistic partners in the aetiology of chronic periodontitis. These two bacterial species intimately associate in polymicrobial biofilms resulting in higher total biomass and show synergistic virulence in animal models of disease. Metabolic exchange can be determinants of interspecies bacterial partnerships. We have shown that coculture of P. gingivalis and T. denticola resulted in upregulation of T. denticola glycine catabolic pathways, while P. gingivalis growth in T. denticola conditioned medium (TdCM) resulted in an increased production of free glycine, which is the preferred energy source for T. denticola. T. denticola and P. gingivalis cross-feeding of succinic acid and isobutyric acid have also been shown.
This study aimed to characterise the mechanism of T. denticola-mediated enhanced free glycine production by P. gingivalis and identify P. gingivalis peptidases involved in the hydrolysis of glycine-containing peptides. Separation of metabolites and peptides in TdCM was performed using tangential flow filtration and reversed phase-high performance liquid chromatography. These fractions were tested against P. gingivalis for their free glycine releasing effects; the results suggested that T. denticola and P. gingivalis peptidases work cooperatively to increase nutrient utilisation efficiency. Mutants of P. gingivalis lacking aminopeptidase (PG1605) or cysteine peptidase (PG1788), displayed reduced free glycine production in TdCM, relative to wild-type. RNA-Seq analysis of P. gingivalis in TdCM indicated a higher level of PG1788 expression. A range of P. gingivalis metabolic pathways were differentially regulated, including significant upregulation of succinate utilisation pathways, supporting the production of succinate by T. denticola for P. gingivalis utilisation. Tripartite multidrug efflux system upregulation by P. gingivalis may indicate T. denticola production of antimicrobial compounds. This work provides insights into the mechanisms involved between P. gingivalis and T. denticola to increase their growth and fitness advantage in a complex community.