This is interesting. I have to wonder if the plaque is indicative of a high population of S.mutans - and that (or the lack of some other biota pushed out) is causing some kind of downstream effect in the microbiome - or if it's the inflammation that occurs in the gums as they pull away from tooth roots that have been coated in plaque.
I also found myself wondering if chewing xylitol gum (something that depresses the S.mutans population in the mouth) might have similar effects. I looked a bit, nothing comparing those and A1C (expectable as that's new) but for plaque control using the gum is not as effective as rinsing with chlorhexidine gluconate or (go figure) Manuka honey - and the honey was more effective in one study https://pubmed.ncbi.nlm.nih.gov/22114423. Honey's a known bacteriocide, so this shouldn't have surprised me so much. No idea if plain raw honey could stand in for the Manuka honey. I've gotten to spend some time in a Manuka tree grove and the smell is just as overwhelming as standing in a grove of Eucalyptus trees (very much not the same smell, just talking intensity here) so maybe there's some key ingredient coming from Manuka blossoms.
Finally S.mutans likes acidic (low pH) saliva, and drymouth, which can happen after radiation treatment or with some psychiatric drugs, drops saliva pH - leading to a higher population of S.mutans. There's a known connection between high A1C (high glucose) and dry mouth - but everything I've seen says that dry mouth is an *effect* of high glucose. Maybe it's not - and maybe it's not only - and is leading to a forward-feedback loop...?
My spellcheck keeps changing 'mutans' into 'Mutants', apologies if any of those have gotten through...