If you find yourself in the backcountry without a toothbrush, you can still practice good dental hygiene: Chew on the end of a stem or twig, then use the gnawed end as a "brush."
Requirements for a stick-brush are simple. It should fray nicely when chewed and be tolerably tasty. Good choices include twigs from sweet gum, pine, birch, and spruce trees, as well as from willow and sage. Just stay away from poison oak or ivy and any tree or bush you can't identify. Many are poisonous to humans when ingested.