Invisible but omnipresent, unappreciated but important, a source of both comfort and discomfort for plants, animals, and people, bacteria underpin the entire living world, from the microscopic to the planetary level, and have been evolving for over three million years. Main contents: 1. In natural history: An invisible world. Bacteria, witness to evolution. Bacteria, driving force of evolution. Bacteria in the tree of life. 2. Recent typing: Archaebacteria. Eubacteria. Cell identity. Identifying bacteria. 3. Parts: Outer shell. Internal structural elements. Genetic material. Supplementary external parts. Genetic language. 4. Vital functions: From genes to proteins. Genetic adaptability. Biological energy. Range of comforts. Eat to live. In numbers and records. With and without oxygen. Increase by multiplying. Adapt to survive. 5. Directed movement: Knowing how to move. Intentional movement. 6. Multiple forms of communication: Genetic communication. Intelligent communication. Communal life. Mutually beneficial associations. 7. Actions and consequences: Pathogens'power. Hosts'natural defences. Bacteria's natural resistance. Some famous names. Emerging and reemerging diseases. Status of bacteria. Bacteria in industry. Bacteria in soil. Bacterial weapons. 8. Permanent evolution: Man as a carrier of bacteria. Bacteria in human history. Future of bacteria. Bacteria on earth. Enter into " bacterreality ". Journey of life. Artistic bacteria