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If you were to completely change your diet from a typical Western diet to a healthy vegan diet, which would involve moving from a very low fibre diet to an extremely high fibre diet then there would be substantial modification in the overall microbiota composition – but generally the adult microbiota community remains stable and resistant to change, since people do not drastically alter their diets.
Even after a course of antibiotics which could kill large numbers of bacteria there would be a rebound to the original baseline state over time, once the antibiotics had been stopped.