There are trillions of microscopic organisms in your gut (digestive system), especially the last part of the gut, the large intestine (colon). They are important for your health. They are sometimes called 'healthy', 'good' or 'friendly' bacteria. They rely on what we eat and drink to grow and to increase in number.
This is a companion discussion topic for the original article at https://patient.info/digestive-health/irritable-bowel-syndrome-leaflet/probiotics-and-prebiotics