We (living creatures) all have beneficial bacteria as well as those not beneficial. If your snake truly needs a bath (which I doubt- & remember they shed now & then too) then plain water is what you use ON the snake, nothing else. Actually, better yet would be to let him slide thru a damp towel a few times- it's not nearly as stressful for a snake as trying to bathe them. Please don't obsess over cleaning a snake- you'll more likely to cause harm.
(UNLESS they have mites, then it's okay to use A DROP of mild liquid soap in the water- but understand, that's not there to clean the snake, but only because it breaks the surface tension so the mites will drown rather than float & survive. It's bad if the snake swallows any soap water- it's likely to cause digestive upsets.)