The results are the opposite of

what would be expected from substrate studies. As mentioned previously, the proteomics shows an increase in the aspartate/asparagine pathway and a reduction in glutamate/glutamine. Culture growth studies found that P. gingivalis grown on aspartylaspartate had significantly more butyrate production than propionate compared to cultures grown on glutamylglutamate [13]. However, a recent flux balance model of P. gingivalis metabolism predicts that there is abundant flexibility in the production of butyrate, propionate and succinate with the metabolic routes to each being equivalent with respect to redox balancing and energy production [20]. Thus a shift towards propionate could be easily explained if it presented an advantage to internalized cells. In that regard, it has been shown that butyrate is a more potent apoptosis inducing agent than propionate GSI-IX [21]. Hence, the diminished production of butyrate by internalized P. gingivalis may contribute to the resistance of P.

gingivalis-infected GECs to apoptotic cell death [22]. There is also the question of the reduced abundance of glutamate BAY 80-6946 dehydrogenase (PGN1367), the protein that converts glutamate to 2-oxoglutarate (Fig. 2). If this is the primary substrate for propionate production it could limit that production even with increased abundance in the rest of the pathway. However, 2-oxoglutarate is a common metabolic intermediate and glutamate/glutamine may not be the only source of 2-oxoglutarate for propionate production. PRKACG Even if it is the primary source, given the flexibility in byproduct production, a significant shift away

from butyrate production from glutamate/glutamine to propionate production could still occur in the presence of an overall reduction in glutamate/glutamine usage. Interestingly, some similar shifts are seen between planktonic cells and biofilms of P. gingivalis strain W50. A mass spectrometry analysis of planktonic cells versus biofilm cells identified 81 proteins and found several energy metabolism proteins with significant differences between planktonic and biofilm lifestyles [23]. In biofilms fumarate reductase (PGN0497, 0498) had reduced abundance while oxaloacetate decarboxylase (PGN0351) had increased abundance similar to what we see in internalized cells (Fig. 2). Obviously, biofilms and the interior of GECs are different environments, and the energy metabolism protein glyceraldehyde-3-phosphate dehydrogenase (PGN0173) was increased in biofilms [23] relative to planktonic cells, while it is decreased in internalized cells relative to external controls. A comparison between the two conditions would really require the identification of more metabolic proteins from biofilm cells, but given the relevance of biofilm formation to P. gingivalis pathogeniCity in vivo [24–26], the relation between biofilm conditions and internalized cells is an interesting one that we intend to pursue further at the whole proteome level.