When we take probiotics, we’re ingesting active good bacteria and yeast in the hope of directly depositing them into the intestines. They do not typically feed existing microflora but instead supply outside help. If the internal environment of your intestines is primed for probiotics, then these organisms multiply, taking root in the microbiome and crowding out harmful bacteria.
The gut microbiome is made up of all kinds of living organisms including bacteria, fungi, viruses, etc. that live inside your intestines. Some aid in the digestion, absorption, and creation of essential nutrients in the body. Others, not so much. And depending on a variety of factors that range from disease to genetics, the composition of that microbiome can change.