Bristle cofounder here. In short, we found that people using an electric toothbrush have increased commensal species in their oral microbiome compared to those using a manual brush. We looked at potential confounders, and found that this effect was negated if you only brush once per day.

We also found that bacterial species that cause bad breath (halitosis) were higher in people using a manual brush, but only people under 30 had reduced scores when using an electric brush. Surprisingly, people with existing gum disease did not benefit as much from an electric toothbrush.

Our data suggest that an electric toothbrush is better at removing biofilm than a manual one. Additionally, accumulation of biofilm seeds an environment for slow-growing anaerobic species. Frequent efficient removal of this biofilm is important for growth of commensal species, and reduction in pathogenic anaerobes that can cause disease.

TLDR: Brush 2x per day, and use an electric toothbrush, especially if you're under 30 and don't have existing gum disease.