The microbiome consists of microbes that are both helpful and potentially harmful.
Most are symbiotic (where both the human body and microbiota benefit) and some, in smaller numbers, are pathogenic (promoting disease). In a healthy body, pathogenic and symbiotic microbiota coexist without problems. But if there is a disturbance in that balance—brought on by infectious illnesses, certain diets, or the prolonged use of antibiotics or other bacteria-destroying medications—dysbiosis occurs, stopping these normal interactions. As a result, the body may become more susceptible to disease.
Microbiota stimulate the immune system, break down potentially toxic food compounds, and synthesize certain vitamins and amino acids, including the B vitamins and vitamin K. For example, the key enzymes needed to form vitamin B12 are only found in bacteria, not in plants and animals.