What role do bacteria play in nature and human life?

Bacteria are the oldest creatures on our planet, and unlike other inhabitants of the Earth, they exist literally everywhere. From the highest layers of the atmosphere to the unexplored depths of the ocean. They play a huge role in nature and in human life. Some bacteria produce oxygen, others – carbon dioxide, sanitary (involved in decomposition), play a role in the circulation of substances. Without them, there would be no fermented milk and fermented products. And in the body of living beings, they play a huge role, including in humans (intestines, etc.) But they also bring harm, cause disease.