Visitors and food are different ideas
Probiotics and prebiotics often sit in the same gut-health conversation, but they do different jobs. Probiotics are live microorganisms added through capsules or fermented foods. Prebiotics are substrates that selected gut microbes can use as food.
A person can take probiotic capsules and still eat very little fibre. That may add visitors without improving the daily food supply in the colon. Fibre-rich foods, resistant starches, some gums, and selected soluble fibres can give microbes material to ferment. This is why a microbiome discussion should not stop at bacteria names. It should also ask what the diet is feeding, repeatedly, across normal meals.