For the gut microbiome, I would recommend avoiding probiotics as they are a complete scam. Instead, all that is needed is proper fiber to provide an environment where good bacteria thrive and bad bacteria get starved out. The easiest and cheapest fiber I've found is chia seed (same as on these pets!) Plus it is packed with omega 3s and has a superior omega 3 to 6 ratio. To get the most benefit, I crack 'em open putting them through the coffee grinder. Just don't let it sit in water too long or it turns into a nasty phlegmish gel (ew!) Also exercise caution and common sense to avoid a poo-splosion, afterall it is fiber.